Human herpesvirus 8 transmission from mother to child and between siblings in an endemic population

BACKGROUND

Transmission of human herpesvirus 8 (HHV-8), the aetiological agent of Kaposi's sarcoma, is known to occur during sex among homosexual men. However, other modes of HHV-8 transmission remain to be elucidated in endemic populations.

METHODS

We did a population-based seroepidemiological survey in a village in French Guiana among 1337 individuals of African origin (age 2-91 years) who had reliable genealogical data. Plasma samples were taken and tested for HHV-specific IgG by immunofluorescence assay. Risk factors and familial correlations for HHV-8 seropositivity were modelled by logistic regression analysis by use of the estimating equations approach, which expresses familial dependences in terms of odds ratios. Familial odds ratios were also acquired by use of the distribution of all possible pairs of a given familial dependence.

FINDINGS

The overall HHV-8 seroprevalence was 13.2% with no difference according to sex. HHV-8 seropositivity was strongly age dependent: at 1.2% under 5 years, HHV-8 seroprevalence rose up to a plateau around 15% between 15 and 40 years, and showed a seroprevalence of more than 27% in individuals older than 40 years. Strong familial aggregation in HHV-8 seroprevalence was found with high mother-child (odd ratio 2.8 [95% CI 1.6-5.0]) and sib-sib (3.8 [1.6-9.5]) correlations. By contrast, no significant correlation between spouses (0.6 [0.2-1.9]) was seen.

INTERPRETATION

This pattern of familial aggregation, together with the variation of HHV-8 seroprevalence with age, indicate that, in endemic populations, HHV-8 transmission mainly occurs from mother to child and between siblings during childhood and adolescence.

The survival motor neuron protein of Schizosacharomyces pombe. Conservation of survival motor neuron interaction domains in divergent organisms

Spinal muscular atrophy is a common often lethal neurodegenerative disease resulting from deletions or mutations in the survival motor neuron gene (SMN). SMN is ubiquitously expressed in metazoan cells and plays a role in small nuclear ribonucleoprotein assembly and pre-mRNA splicing. Here we characterize the Schizosacharomyces pombe orthologue of SMN (yeast SMN (ySMN)). We report that the ySMN protein is essential for viability and localizes in both the cytoplasm and the nucleus. Like human SMN, we show that ySMN can oligomerize. Remarkably, ySMN interacts directly with human SMN and Sm proteins. The highly conserved carboxyl-terminal domain of ySMN is necessary for the evolutionarily conserved interactions of SMN and required for cell viability. We also demonstrate that the conserved amino-terminal region of ySMN is not required for SMN and Sm binding but is critical for the housekeeping function of SMN.

Detection of a major gene predisposing to human T lymphotropic virus type I infection in children among an endemic population of African origin

Human T lymphotropic virus type I (HTLV-I) is a human oncoretrovirus that causes an adult T cell leukemia/lymphoma and a chronic neuromyelopathy. To investigate whether familial aggregation of HTLV-I infection (as determined by specific seropositive status) could be explained in part by genetic factors, we conducted a large genetic epidemiological survey in an HTLV-I-endemic population of African origin from French Guiana. All of the families in 2 villages were included, representing 83 pedigrees with 1638 subjects, of whom 165 (10.1%) were HTLV-I seropositive. The results of segregation analysis are consistent with the presence of a dominant major gene predisposing to HTLV-I infection, in addition to the expected familial correlations (mother-offspring, spouse-spouse) due to the virus transmission routes. Under this genetic model, approximately 1. 5% of the population is predicted to be highly predisposed to HTLV-I infection, and almost all seropositive children <10 years of age are genetic cases, whereas most HTLV-I seropositive adults are sporadic cases.

Linkage analysis of quantitative trait loci: sib pairs or sibships?

Sib pair linkage studies are now widely used to investigate the genetic factors implicated in complex quantitative traits. To increase the power of these approaches, it has been proposed to select extremely discordant (ED) sib pairs which are expected to contain the highest linkage information. However, it is known that sibships of larger size contain more linkage information than independent sib pairs. In this paper we compare, in terms of power and cost considerations, the ED strategy, which uses information on sib pairs only, to the recently developed 'Maximum Likelihood Binomial' sibship-oriented method performed on the whole sibships from which the ED sib pairs have been extracted. We show that the use of these whole sibships is an efficient alternative to approaches focusing on ED sib pairs only.

Comparison of four sib-pair linkage methods for analyzing sibships with more than two affecteds: interest of the binomial maximum likelihood approach

Family samples collected for sib-pair linkage studies usually include some sibships with more than two affecteds (multiplex sibships). Several methods have been proposed to take into account these multiplex sibships, and four of them are discussed in this work. Two methods, which are the most widely used, are based on the number of alleles shared by the sib-pairs constitutive of the multiplex sibship, with the first using the total number of these shared alleles ("all possible pairs" method) and the second considering a weighted number of these alleles (weighted method). The two other approaches considered the sibship as a whole, with in particular a likelihood method based on a binomial distribution of parental alleles among affected offspring. We theoretically show that, in the analysis of sibships with two affecteds, this likelihood method is expected to be more powerful than the classical mean test when a common asymptotic type I error is used. The variation of the sibship informativeness (assessed by the proportion of heterozygous parents) according to the number of affected sibs is investigated under various genetic models. Simulations under the null hypothesis of no linkage indicate that the "all possible pairs" is anticonservative, especially for type I errors < or = 0.001, whereas the weighted method generally provides satisfactory results. The likelihood method shows very consistent results in terms of type I errors, whatever the sample size, and provides power levels similar to those of the other methods. This binomial likelihood approach, which accounts in a natural way for multiplex sibships and provides a simple likelihood-ratio test for linkage involving a single parameter, appears to be a quite interesting alternative to analyze sib-pair studies.

Introduction of the IBD information into the weighted pairwise correlation method for linkage analysis

The Weighted Pairwise Correlation (WPC) approach is a non-parametric method of linkage analysis that allows analysis of any kind of phenotypes (quantitative, binary, binary with age of onset) and to consider all pairs of relatives in a pedigree. The principle of this method is to test whether two relatives having close phenotypes also resemble at the marker locus more than expected under the null hypothesis of no linkage. So far, this marker resemblance was estimated by the proportion of alleles shared Identical By State (IBS) by the two relatives. Here, we propose a method to incorporate the Identical By Descent (IBD) information into the WPC approach. For any kind of relative pairs, the computation of the proportion of alleles shared IBD is based on the identification of the closest couple of ancestors, denoted as the reference couple. The IBD information is obtained for pairs of relatives having the same reference couple using individual genotypic vectors derived from this couple. This reconstruction of the IBD information is performed rapidly even in large pedigrees. Simulation studies conducted under various genetic models demonstrate that the use of IBD instead of IBS information leads to a large increase of power, especially in the situation of poorly informative markers.

Human malaria: segregation analysis of blood infection levels in a suburban area and a rural area in Burkina Faso

The genetic control of blood infection levels in human malaria remains unclear. Case control studies have not demonstrated a strong association between candidate genes and blood parasite densities as opposed to surveys that have focused on severe malaria. As an alternative approach, we used segregation analyses to determine the genetic control of blood parasitemia. We surveyed 509 residents (53 pedigrees) in a rural area and 389 residents (41 pedigrees) in an urban area during 18 months. Each family was visited 20 times and 28 times in the urban area and in the rural area; the mean number of parasitemia measurements per subject was 12.1 in the town and 14.9 in the village. The intensity of transmission of Plasmodium falciparum was 8-fold higher in the rural area than in the urban area. Using the class D regressive model for both populations, we found that blood parasite densities were correlated between sibs. We obtained strong evidence for a major effect, but we found that the transmission of this major effect was not compatible with a simple Mendelian model, suggesting a more complex mode of inheritance. Moreover, there was a strong interaction between major effect and age, suggesting that the influence of the putative major gene may be more prominent in children than in adults. Further nonparametric linkage studies, such as sib pair analysis, that focus on children would help us better understand the genetic control of blood infection levels.

Two-locus developments of the weighted pairwise correlation method for linkage analysis

Different studies of complex traits assumed to be influenced by two unliked loci found that two-locus linkage analysis is more powerful than the classical one-locus strategy. The Weighted Pairwise Correlation (WPC) approach is a nonparametric method for linkage analysis that has the advantage to analyze any kind of phenotypes and to consider extended pairs of relatives. In this report, we propose different two-locus extensions of the WPC method based on an additive or a multiplicative effect of two unlinked marker loci on the phenotype. Both methods and their corresponding statistics are easily derived from the classical WPC approach. Compared to the additive model, the multiplicative approach, which can be understood as a statistical interaction effect of the two markers, does not need to specify any additional parameter and allows one to test both the global effect of the two markers (T(AB)test) and the effect of one marker, e.g., B, taking into account the effect of the other, A (T(AB/A) test). When compared to classical one-locus tests by means of simulations, two-locus tests have comparable 0.05 type I error and are more powerful. In particular, tests based on the multiplicative approach appear to be quite interesting in addition to single locus tests to detect the combined role of two markers (T(AB)), or to investigate the role of a marker taking into account a known linked marker (T(AB/A)), especially when these markers have complex effects on the phenotype (e.g., statistical interaction).

Maximum-Likelihood-Binomial method for genetic model-free linkage analysis of quantitative traits in sibships

Sib-pair linkage studies are widely used to investigate the genetic factors implicated in complex quantitative traits. To analyze these data, we propose a Maximum-Likelihood-Binomial (MLB) approach, which considers the sibship as a whole and relies on the idea of binomial distributions of parental alleles among offsprings. The method is based on the introduction of a latent binary variable capturing the linkage information between the observed quantitative trait and the marker, and the final likelihood can be expressed assuming a parametric distribution for the studied trait but also without any assumption on this distribution. The test for linkage is a simple likelihood ratio test involving a single parameter. The performances of the MLB method are assessed by a simulation study in different kinds of family samples. In the case of families with various sibship sizes, both MLB approaches (assuming or not a parametric distribution for the quantitative trait) provide very consistent results in terms of type I errors and yield power levels generally higher than those of the classical Haseman-Elston method. In the case of extremely discordant sib pairs, we analytically show that, for a common asymptotic type I error, the distribution-free MLB statistic is expected to be more powerful than the test proposed by Risch and Zhang [(1995) Science 268:1584-1589]. In samples including both extremely concordant and discordant sib-pairs, simulation studies show that the MLB approach is at least as powerful as the EDAC method [Gu et al. (1996) Genet Epidemiol 13:513-533]. This MLB method, which can be easily extended to perform multipoint analysis and to account for genetic heterogeneity, appears to be quite an interesting alternative for mapping quantitative trait loci in humans.

Granulomatous reaction to intradermal injection of lepromin (Mitsuda reaction) is linked to the human NRAMP1 gene in Vietnamese leprosy sibships

The Mitsuda test, which measures the specific immune response against intradermally injected lepromin, has a high prognostic value for susceptibility or resistance to the lepromatous form of leprosy. A sib-pair linkage analysis between the Mitsuda response and the NRAMP1 gene was done among 20 nuclear families with leprosy (totaling 118 sibs) from Ho Chi Minh City, Vietnam. All family subjects were genotyped for several intragenic and flanking NRAMP1 markers, leading to the definition of a fully informative NRAMP1 haplotype. Significant linkage was observed between NRAMP1 and Mitsuda reaction when considered either as a quantitative (P<.002) or as a categorical (P=.001) trait. Separate analyses among healthy and affected sibs showed evidence for linkage in both subsamples, indicating that linkage between the Mitsuda reaction and NRAMP1 is independent of leprosy status. These results support the view that NRAMP1 plays a regulatory role for the development of acquired antimycobacterial immune responses as determined by in vivo Mitsuda test reaction.

[Genetic predisposition to bilharziasis in humans: research methods and application to the study of Schistosoma mansoni infection]

The development of genetic epidemiology methods using recent human genetic mapping information together with the growing availability of candidate genes has led to major advances in the identification of host genes in human schistosomiasis. Two phenotypes have been studied so far in the infection by Schistosoma mansoni: infection levels by the parasite as measured by the faecal egg counts, and the severe hepatic fibrosis caused by S. mansoni assessed by ultrasound examination. The first study was performed on Brazilian pedigrees and provided strong evidence for a major gene controlling infection levels by S. mansoni denoted as SM1 which was mapped to chromosome 5q31-q33. This region contains several candidate genes involved in the regulation of the Th1/Th2 response, and the direct role of polymorphisms located within these genes is under investigation. The second study conducted in Sudan also showed the presence of a major gene influencing the development of severe hepatic fibrosis due to S. mansoni infection denoted as SM2. This gene is not located in the 5q31-q33 region, but maps to chromosome 6q22-q23 and is closely linked to the IFN-gamma R1 gene encoding the receptor of the strongly anti-fibrogenic cytokine Interferon-gamma. These findings indicate that two distinct genetic loci control human predisposition to schistosomiasis, SM1 located in the 5q31-q33 region which is likely to play a role in the Th1/Th2 differentiation, and SM2 in 6q22-q23 influencing disease progression with a possible involvement in the regulation of IFN-gamma.

Genetic model-free linkage analysis using the maximum-likelihood-binomial method for categorical traits

Within the simulated data of the 11th Genetic Analysis Workshop, we searched for the genes controlling the disease. We analyzed 200 families from Studies 2 and 3 presenting both mild and severe forms of disease. Linkage analysis was performed using the recently developed genetic model-free maximum-likelihood-binomial (MLB) method which overcomes the problem of multiple sibs by considering the sibship as a whole. The MLB allowed us to consider the disease as either a binary (affected/unaffected) or an ordered categorical (differentiating the two forms of disease and including effects of environmental factors) phenotype. In both studies, two regions provided evidence for linkage at a significance level below 10(-4). One is located on chromosome 3 (from D3G041 to D3G047), and the other on chromosome 5 (from D5G034 to D5G041). In Study 2, the most significant results were obtained by combining both forms of disease, suggesting that they are under the same genetic control, while in Study 3, the stronger results were obtained when considering severe subjects alone, suggesting that only the severe form is under the control of both locus B and C. The subsequent knowledge of the true model allowed a posterior interpretation of our results, in particular the difference in optimal coding schemes observed between Studies 2 and 3, and the failure to locate locus A.

Genome search for alcohol dependence using the weighted pairwise correlation linkage method: interesting findings on chromosome 4

The nonparametric (model-free) method of linkage analysis Weighted Pairwise Correlation (WPC) has been proposed by Commenges [1994], and extended to incorporate identity-by-descent (IBD) information [Zinn-Justin and Abel, 1999]. We performed an autosome-wide scan in the Collaborative Study on the Genetics of Alcoholism data using the WPC-IBD method, considering two phenotypes related to alcohol dependence defined as residuals of binary traits adjusted for age and sex. Three chromosome 4 markers located in a region of 50 cM spanning from GABRB1 to D4S1651 provided Monte Carlo (MC) p-values lower than 0.005, confirming the possible influence of beta 1 GABA receptor and ADH genes in alcoholism. Furthermore, marker D15S642, not far from the ALDH6 gene, provided an MC p-value of 0.0005 in ethnic groups "White, Hispanic" and "White, non-Hispanic."

Susceptibility to periportal (Symmers) fibrosis in human schistosoma mansoni infections: evidence that intensity and duration of infection, gender, and inherited factors are critical in disease progression

Lethal disease in Schistosoma mansoni infections is mostly due to portal hypertension caused by hepatic periportal fibrosis. To evaluate the factors that may determine severe disease, livers and spleens were examined by ultrasound in a Sudanese population living in a village where S. mansoni is endemic. Early (FI), moderate (FII), or advanced (FIII) fibrosis was observed in 58%, 9%, and 3% of the population, respectively. Although FI affected 50%-70% of the children and adolescents, FII prevalence was low in subjects </=20 years old but increased sharply (45%-58%) in men 21-30 years old and was associated with the highest infections. Portal and splenic vein diameters were increased in one-third of persons with FII and in almost all with FIII disease. Severe disease, FII or FIII with portal hypertension, affected 6% of the population, was associated with splenomegaly, occurred mostly in adult men, and was clustered in a few pedigrees. These observations suggest that infection intensity and duration, gender-related factors, and inherited factors are important in fibrosis development.

Severe hepatic fibrosis in Schistosoma mansoni infection is controlled by a major locus that is closely linked to the interferon-gamma receptor gene

Lethal disease due to hepatic periportal fibrosis occurs in 2%-10% of subjects infected by Schistosoma mansoni in endemic regions such as Sudan. It is unknown why few infected individuals present with severe disease, and inherited factors may play a role in fibrosis development. Schistosoma mansoni infection levels have been shown to be controlled by a locus that maps to chromosome 5q31-q33. To investigate the genetic control of severe hepatic fibrosis (assessed by ultrasound examination) causing portal hypertension, a segregation analysis was performed in 65 Sudanese pedigrees from the same village. Results provide evidence for a codominant major gene, with.16 as the estimated allele A frequency predisposing to advanced periportal fibrosis. For AA males, AA females, and Aa males a 50% penetrance is reached after, respectively, 9, 14, and 19 years of residency in the area, whereas for other subjects the penetrance remains <.02 after 20 years of exposure. Linkage analysis performed in four candidate regions shows that this major locus maps to chromosome 6q22-q23 and that it is closely linked (multipoint LOD score 3.12) to the IFN-gammaR1 gene encoding the receptor of the strongly antifibrogenic cytokine interferon-gamma. These results show that infection levels and advanced hepatic fibrosis in human schistosomiasis are controlled by distinct loci; they suggest that polymorphisms within the IFN-gammaR1 gene could determine severe hepatic disease due to S. mansoni infection and that the IFN-gammaR1 gene is a strong candidate for the control of abnormal fibrosis observed in other diseases.

[Genetic epidemiology in the study of susceptibility/resistance to malaria in the human population]

The development of genetic epidemiology methods using recent human genetic map together with the growing availability of candidate genes have led to substantial advances in the identification of host genes in human malaria. Investigation of these genes has progressed along two complementary ways: 1) The search for genes influencing the severe malaria clinical phenotype by means of population based case-control studies which showed the protective role of several red cell genetic defects (sickle cell anemia, a-thalassaemia ...) and that some polymorphisms of the TNF-alpha promoter region could predispose to cerebral malaria; 2) The investigation of the genetic regulation of malaria-related biological phenotypes (infection levels, immune response) by means of familial studies which underlined the influence of the 5q31-q33 chromosomal region in the control of Plasmodium falciparum blood parasitemia and the role of major histocompatibility complex (MHC) and non-MHC genes in the regulation of humoral and cellular response to various malarial antigens. Ongoing studies will precise the role of these genes and probably reveal the existence of other genes not identified yet. The impact of these findings on the understanding of malaria pathogenesis and on the design of future preventive and therapeutic strategies should be considerable.

Genetic dissection of Plasmodium falciparum blood infection levels and other complex traits related to human malaria infection

There is accumulating evidence of host genetic control in malaria infection and, in humans, some genes have been associated with severe malaria. Nevertheless, other important genes controlling blood infection levels, malarial disease and immune responses are likely to be identified. In this paper, we focus on segregation and linkage analyses of blood infection levels in an urban population living in Burkina Faso. We found evidence of a complex genetic control and a linkage to chromosome 5q31-q33. The identification of genes controlling complex traits related to malaria infection should be helpful in understanding protective mechanisms and the relationship between infection, malaria attacks and severe malaria.

Genetic epidemiology of host predisposition microfilaraemia in human loiasis

Evidence is accumulating from experimental and human studies that genetic factors are involved both in the control of infectious diseases and in the regulation of infection levels and clinical presentation. So far few studies have investigated the role of these genetic factors in human infection by the filarial parasite Loa loa. We present a segregation analysis on 74 nuclear families who live in the tropical rainforest of southern Cameroun and are exposed to homogeneous loiasis transmission. The results indicate that there is a genetic predisposition to be microfilaraemic and that predisposed subjects might be genetically unable to mount an efficient immune response against loiasis antigens. This individual susceptibility could explain at least in part why the prevalence of infection (microfilaraemic individuals) does not usually exceed 30% of the exposed population in hyperendemic regions. Further genetic studies, based on linkage analysis using both familial information and genetic markers, will help to identify the nature of the genetic factors predisposing to microfilaraemia.

Differential effects of CD4+ and CD8+ cells on lymphocyte development from human cord blood cells in murine fetal thymus explants

The possibility that mature lymphocytes play a role in the regulation of human T cell development was studied in the experimental model of fetal thymus organ cultures (FTOC), by reconstituting lymphocyte-depleted murine fetal thymus (FT) lobe with cells isolated from human umbilical cord blood (CB). Cultures were incubated with human cytokines (IL-7, FLT-3 ligand and Steel Factor), or remained untreated. When CD4+, or CD8+ CB cells, were co-cultured with FT explants, they expanded and maintained their original phenotypic markers, with no significant effect of the cytokines. Cultures of human hematopoietic stem cells (CD34+) gave rise to CD4+CD8- cells, which were mainly CD3-, with no indication of further intermediate developmental stages. However, a limited number of CD4+CD8+ (double positive [DP]) cells were detected when the CD34- cells were co-cultured with CD4+ cells from the same CB samples. In contrast, FT with unseparated CB cells resulted in the different CD4/CD8 subsets, and their numbers increased in the presence of cytokines. The appearance of DP cells depended on the presence of either CD4+ or CD8+ cells in the cultured CB samples. Hence, DP cells were not detected when the CB was depleted of CD4+ and CD8- cells ("depCB") before culture, and they appeared when depCB were co-cultured with either CD4+ or CD8+ cells. In contrast, CD4+ cells inhibited the development of CD8+CD3+ cells, and this was most pronounced in the absence of the cytokines. There was no symmetrical down-regulatory effect of CD8+ cells on the development of CD4+CD3+ cells. Addition of IL-15 to the cytokine mixture led to an increased proportion of CD56+ cells in cultures of CD34+ cells. The presence of CD4+, and not CD8+ cells, interfered with this process. Our results thus imply differential effects of CD4+ and CD8+ cells on thymocytopoiesis.

[Analysis of the genetic factors controlling malarial infection in man]

Genetic factors have clearly been shown to play a role in controlling malarial infection in animal models. There is now also increasing evidence for the genetic control of malaria in man. We carried out a segregation analysis based on blood parasite load phenotype for a population of the town of Bobo-Dioulasso (Burkina-Faso). This analysis demonstrated a strong genetic effect. Our results were not consistent with the segregation of a major gene and thus suggest that parasite load is under the control of minor genes. The genetic effect was stronger in children than in adults. We carried out a regression analysis in children and found that there was an association between the phenotype for blood parasite load and the q31-33 region of chromosome 5. We identified a gene in this region, Pfil1 (Plasmodium falciparum infection levels 1), which accounted for almost 50% of the variance in blood parasite load and which played a fundamental role in the control of infection. The 5q31-33 region contains several genes encoding cytokines that regulate T lymphocytes. The identification of genes controlling malarial infection opens up new possibilities for preventive and treatment strategies. It should be possible in the near future to identify individuals at risk of malaria, who would derive the greatest benefit from preventive and therapeutic measures. Finally, a deeper understanding of these genes controlling protective immune responses could be of value for the development of vaccines.

Full results of the genome-wide scan which localises a locus controlling the intensity of infection by Schistosoma mansoni on chromosome 5q31-q33

Three hundred million individuals are at risk of infection by schistosomes, and thousands die each year of severe hepatic disease. Previous studies have shown that the intensity of infection by Schistosoma mansoni in a Brazilian population is controlled by a major gene, denoted as SM1. We report here the full results of a genome-wide search that was performed on this population to localise SM1. Two hundred and forty-six microsatellites were used for the primary map, and only one region in 5q31-q33 provided significant evidence of linkage. SM1 was subsequently mapped to this region, which contains several genes encoding cytokines or cytokine receptors which are involved in protection against schistosomes. Three additional regions, 1p22.2, 7q36 and 21q22-22-qter, yielded promising, although not significant, lod-score values. These regions contain candidate genes encoding cytokines or molecules relevant to anti-schistosome immunity.

Robustness and power of the maximum-likelihood-binomial and maximum-likelihood-score methods, in multipoint linkage analysis of affected-sibship data

The maximum-likelihood-binomial (MLB) method, based on the binomial distribution of parental marker alleles among affected offspring, recently was shown to provide promising results by two-point linkage analysis of affected-sibship data. In this article, we extend the MLB method to multipoint linkage analysis, using the general framework of hidden Markov models. Furthermore, we perform a large simulation study to investigate the robustness and power of the MLB method, compared with those of the maximum-likelihood-score (MLS) method as implemented in MAPMAKER/SIBS, in the multipoint analysis of different affected-sibship samples. Analyses of multiple-affected sibships by means of the MLS were conducted by consideration of all possible sib pairs, with (weighted MLS [MLSw]) or without (unweighted MLS [MLSu]) application of a classic weighting procedure. In simulations under the null hypothesis, the MLB provided very consistent type I errors regardless of the type of family sample (sib pairs or multiple-affected sibships), as did the MLS for samples with sib pairs only. When samples included multiple-affected sibships, the MLSu led to inflation of low type I errors, whereas the MLSw yielded very conservative tests. Power comparisons showed that the MLB generally was more powerful than the MLS, except in recessive models with allele frequencies <.3. Missing parental marker data did not strongly influence type I error and power results in these multipoint analyses. The MLB approach, which in a natural way accounts for multiple-affected sibships and which provides a simple likelihood-ratio test for linkage, is an interesting alternative for multipoint analysis of sibships.