Synteny mapping of the bovine IGHG2, CRC and IGF1 genes

A panel of bovine-murine hybrid cell lines was analysed for 10 loci, including three (IGF1, IGHG2 and the calcium release channel gene [CRC]) that have previously been mapped in man, but not in cattle. The IGF and CRC genes were indirectly mapped to chromosomes 5 and 18 respectively and the syntenies of the HOX2 and GH genes and of the NP and FOS genes were confirmed. The results also show that the IGHG2 locus, which is linked to NP and FOS on human chromosome 14, is separated from these genes in cattle. By showing synteny of the IGHG2 and MPI loci, the IGHG2 locus has been indirectly mapped to chromosome 21.

Stage-specific transcription of germline IgH C gamma and C alpha regions during human B cell differentiation

Previous studies have suggested that transcription of germline heavy chain constant region (CH) genes in murine B cells may determine the potential of their different CH regions to undergo isotype switch recombination. We have examined the transcriptional activity across the immunoglobulin heavy chain (IgH) locus in human B lineage cells. Transcription of germline C gamma and C alpha was observed in every surface IgM+ or surface IgM+/IgD+ B cell stage cell line and malignancy. In contrast, such transcription could not be detected in pre-B cells and only low levels of C alpha but not C gamma transcription were evident in IgM-secreting plasmablast cells. Transcriptional activity of germline IgH C epsilon was singularly absent at all stages of B cell development. Our results suggest that germline transcription of the C gamma and C alpha regions may be a constitutive feature of the human B cell differentiation program. Because this transcriptional activity is limited primarily to the B cell stage and occurs prior to the actual isotype switch, the induction of C gamma and C alpha transcription may represent preparation of the downstream IgH chromatin for potential switch recombination.

Juvenile chronic arthritis and juvenile spondyloarthropathy

Clinically recognized subgroups of juvenile chronic arthritis and the juvenile spondyloarthropathies are gradually being shown to be immunogenetically distinct; greater subdivision may ultimately be required. Mechanisms by which the association of certain genes work await further elucidation. Meanwhile, therapy such as intravenous gamma globulin may be most effective in systemic disease. Sulfasalazine is recommended for patients with later-onset pauciarticular disease, particularly HLA-B27 patients. Methotrexate should probably be reserved for severe unresponsive disease, particularly polyarthritis, until side effect profiles are better evaluated.

Characterization of the enhancer region for germline transcription of the gamma 3 constant region gene of human immunoglobulin

A constant region gene (C) of the immunoglobulin heavy chain can be transcribed as germline transcripts from a promoter located upstream of a switch region. We have studied the structure and function of the human C gamma 3 promoter region. When the human IgM-producing cell line SSK41 is stimulated with interleukin 4 (IL-4) in the presence of phorbol myristate acetate (PMA) or Staphylococcus aureus Cowan I, expression of germline C gamma 3 transcripts was specifically augmented within 4 h. Upstream DNA fragments flanking the I gamma 3 exon were fused with a reporter gene and tested for IL-4-induced promoter/enhancer activity by transfection of SSK41 cells. The DNA fragment between 450 and 250 bp upstream of the transcription initiation site of the C gamma 3 gene was shown to be required for transcriptional up-regulation by PMA and IL-4. The upstream 514 bp fragment of the I gamma 3 flanking region was shown to contain an enhancer activity in response to PMA and IL-4.

A genetic study of retinopathy in south Indian type 2 (non-insulin-dependent) diabetic patients

Genetic marker studies in diabetic retinopathy are controversial and frequently complicated by possible independent associations of Type 1 (insulin-dependent) diabetes mellitus with the markers so far analysed. We have looked for associations of candidate genes with retinopathy in South Indian Type 2 (non-insulin-dependent) diabetic patients; patients were subdivided into those with exudative maculopathy (n = 53), proliferative retinopathy (n = 40) and patients free from diabetic retinopathy with a minimum disease duration of 15 years (n = 45). DNA was extracted from blood samples and studied by Southern blot hybridisation techniques and the following probe enzyme combinations: HLA-DQB1; Taq 1, HLA-DQA1; Taq 1, HLA-DRA; Bgl II, insulin gene hypervariable region; Pvu II and the switch region of the immunoglobulin IgM heavy chain gene (S mu); Sac I. Differences in genotype distributions between the study groups were only detected with the S mu probe which detects polymorphism of both S mu and S alpha 1 (the switch region of IgA). Two alleles of S alpha 1 were detected sized 7.4 kilobase and 6.9 kilobase. The frequency of 6.9 kilobase homozygotes was lower in proliferative retinopathy (19%) compared to patients free from diabetic retinopathy (54%, p = 0.005) and exudative maculopathy (46%, p = 0.03). This data suggests that there is a genetic predisposition to proliferative retinopathy in Type 2 (non-insulin-dependent) diabetes of South Indian origin and that this is determined by polymorphism of the heavy chain immunoglobulin genes located on chromosome 14.

Differential mRNA stabilities affect mRNA levels in mutant mouse myeloma cells

A series of mouse myeloma cell lines producing mutant gamma 2b immunoglobin heavy chains, which resemble heavy chain disease proteins, were analyzed for messenger RNA abundance as a function of mRNA alterations. A mutation effectively deleting the gamma 2b-CH1 domain of the mRNA had little or no effect on Ig heavy chain mRNA abundance on half-life (mutant 10.1). A mutation in the gamma 2b-CH2 and CH3 domain, causing premature termination of translation, had more deleterious effects on Ig heavy chain mRNA abundance and half-life (mutant I17). Substitution of the deleted portions of the gamma 2b mRNA with gamma 2a sequences by subclass switching in the cells (mutants K23 and K25) resulted in increased heavy chain abundance and half-life relative to the parent I17. In contrast, kappa light chain mRNA levels and half-lives remain constant among the mutants. The wild-type and mutant cell lines transcribed the Ig heavy chain gamma 2b locus equally when compared with an internal beta-actin standard by transcription run on studies. Therefore, half-life of the Ig heavy chain mRNA seems to be the principal determinant in cytoplasmic mRNA abundance in this system.

Antigen processing by epidermal Langerhans cells correlates with the level of biosynthesis of major histocompatibility complex class II molecules and expression of invariant chain

Two prior studies with a small number of T cell lines have shown that the presentation of native protein antigens by epidermal Langerhans cells (LC) is regulated. When freshly isolated, LC are efficient antigen-presenting cells (APC), but after a period of culture LC are inefficient or even inactive. The deficit in culture seems to be a selective loss in antigen processing, since cultured LC are otherwise rich in major histocompatibility complex (MHC) class II products and are active APC for alloantigens and mitogens, which do not require processing. We have extended the analysis by studying presentation to bulk populations of primed lymph node and a T-T hybrid. Only freshly isolated LC can be pulsed with the protein antigens myoglobin and conalbumin, but once pulsed, antigen is retained in an immunogenic form for at least 2 d. The acquisition of antigen, presumably as MHC-peptide complexes, is inhibited if the fresh LC are exposed to foreign protein in the presence of chloroquine or cycloheximide. The latter, in contrast, improves the efficacy of antigen pulsing in anti-Ig-stimulated B blasts. In additional studies of mechanism, we noted that both fresh and cultured LC endocytose similar amounts of an antigen, rhodamineovalbumin, into perinuclear granules. However, freshly isolated LC synthesize high levels class II MHC molecules and express higher amounts of the class II-associated invariant chain. Fresh LC are at least 5-10 times more active than many other cells types in the level of biosynthesis of MHC class II products. These findings provide a physiologic model in which newly synthesized MHC class II molecules appear to be the principal vehicle for effective antigen processing by APC of the dendritic cell lineage. Another APC, the B lymphoblast, does not appear to require newly synthesized MHC class II molecules for presentation.

Gene segments encoding membrane domains of the human immunoglobulin gamma 3 and alpha chains

The carboxyterminal region of the heavy chains, according to its hydrophilic or hydrophobic properties, determines whether the immunoglobulin will be secreted or membrane-bound. We have determined the nucleotide sequences of the human IGHG3, IGHA1, and IGHA2 membrane exons isolated from genomic DNA libraries. The IGHG3 M1 and M2 exons are separated by a long intron of 2.1 kilobases (kb) containing an highly repeated motif of 34 base pairs (bp). The IGHA1 and IGHA2 genes, like the mouse Igh-A gene, have a single exon encoding the extracellular, transmembrane, and cytoplasmic regions. For each class of immunoglobulins, the sequences of membrane exons are highly conserved between human and mouse, but no alignment is possible for the flanking regions. In contrast, for a same species, the sequences of the heavy chain membrane exons differ from one class to another. While the hydrophobic profile of the membrane core is well conserved, the cytoplasmic region differs in length and in composition. None of the intracellular domains presents the sequence implied in signal transduction, implying that membrane immunoglobulins need other proteins, which probably interact with the constant or membrane domain, to transmit signals leading to B-cell activation.

Individual VH genes detected with oligonucleotide probes from the complementarity-determining regions

The germ-line and expressed Ig repertoire was examined with three oligonucleotide probes from the CDR regions of VH18/2, a VH gene from the largest human VH gene family, VHIII. Each oligonucleotide probe detected small numbers of germ-line bands (1-5) under conditions in which single base differences can be detected; more than half of these bands were polymorphic. The combined results from pairs of oligonucleotides from CDR1 and CDR2 identified a single band on Southern blots, as did a probe from the 5' end of CDR2. This band contains the 18/2 germ-line gene. The nucleotide sequence of expressed VH genes that hybridized to both CDR probes or to the 5' CDR2 probe were greater than or equal to 97% homologous to 18/2 in both the framework and CDR regions. This group of closely related VH genes, the 18/2 CDR family, appears to be overexpressed. The role of polymorphisms and differential expression of individual V genes in multigenic autoimmune diseases, as well as the organization and expression of individual V genes, can be examined with pairs of oligonucleotides from CDR1 and the 3' end of CDR2, or with probes from the 5' end of CDR2.

Relationship between immunoglobulin allotypes and susceptibility to nasopharyngeal carcinoma in Malaysia

The relationship between immunoglobulin allotypes and risk of developing nasopharyngeal carcinoma was examined in a comparative study of 50 Chinese cases and 140 Chinese controls and 50 Malay cases and 79 Malay controls residing in Malaysia. Although the most common Gm phenotype was elevated in both Chinese and Malay nasopharyngeal carcinoma patients compared to their controls, there were no significant differences between cases and controls in the distribution of Gm haplotypes in either population. There were no differences between cases and controls in the distribution of Km alleles in either population. Thus a previously reported association of Km(1) with increased nasopharyngeal carcinoma risk in Tunisia is not confirmed in two Mongoloid populations in Malaysia.

Myelomas and lymphomas expressing the Ig gamma 2a H chain gene have similar transcription termination regions

During B cell differentiation, the membrane and secretion specific forms of the Ig gamma-H chains of mouse are differentially expressed as a function of the developmental stage of the cell. Representatives of less differentiated and memory B cells (lymphomas) that have undergone the class switch to gamma 2a or gamma 2b H chains produce nearly equal amounts of membrane specific (gamma m) vs secretory specific (gamma s) mRNA. Fully differentiated gamma 2a or gamma 2b plasma cells and their tumors, myelomas, switch to higher levels of gamma s mRNA production relative to gamma m. Selective use of either the gamma s poly(A) site or the downstream gamma m poly(A) site accompanied by specific splicing events could modulate production of these two forms of mature gamma H chain mRNA. Alternatively, transcription termination could be modulated. Through a combination of hybrid protection and in vitro nascent RNA analyses of transcripts from gamma H chain-producing cells arrested at various stages of development, we have mapped transcription termination in both lymphomas (gamma s approximately gamma m mRNA) and in myelomas (gamma s much greater than gamma m) mRNA. Regardless of the developmental stage of the cell, transcription proceeds at a significant level through both the secretory- and membrane-specific poly(A) sites and terminates at least 500 nucleotides downstream of the gamma m poly(A) site in both the gamma 2a and gamma 2b genes. We conclude that transcription termination does not play a major role in the switch to elevated levels of gamma s production in late stage gamma-producing myeloma cells and that alternative RNA processing alone must be responsible for the differential expression of the gamma H chain mRNA.

Expression of heterobispecific antibodies by genes transfected into producer hybridoma cells

We report the expression of heterobispecific antibodies (Ab) by transferred genes. The kappa and gamma 1 genes of a mouse anti-idiotypic Ab (IgG1) were transfected into a mouse hybridoma cell line secreting Ab (IgG1), directed against an isoenzyme of human creatine kinase. Stable cell lines secreting the parental Ab derived from the introduced genes and a mixture of hybrid Ab were established. The transfected Ab specificity was expressed at similar levels as in a nonproducer background (50 ng-1 microgram/ml), heterobispecific Ab was expressed in microgram quantities (1-4 micrograms/ml) in all cell lines examined. As shown by isoelectric focusing analysis, hybrid Ab (heterobifunctional and other species) are expressed to a similar extent in the transfected cell lines as the Ab in the parental Ab-producing cells.

Immunoglobulin heavy-chain enhancer is required to maintain transfected gamma 2A gene expression in a pre-B-cell line

The immunoglobulin heavy-chain (IgH) enhancer serves to activate efficient and accurate transcription of cloned IgH genes when introduced into B lymphomas or myelomas. The role of this enhancer after gene activation, however, is unclear. The endogenous IgH genes in several cell lines, for example, have lost the IgH enhancer by deletion and yet continue to be expressed. This might be explained if the role of the enhancer were to establish high-level gene transcription but not to maintain it. Alternatively, other enhancers might lie adjacent to endogenous IgH genes, substituting their activity for that of the lost IgH enhancer. To address both of these alternatives, we searched for enhancer activity within the flanking regions of one of these IgH enhancer-independent genes and designed an experiment that allowed us to consider separately the establishment and maintenance of expression of a transfected gene. For the latter experiment we generated numerous pre-B cell lines stably transformed with a gamma 2a gene. In this gene, the IgH enhancer lay at a site outside the heavy-chain transcription unit, between DH and JH gene segments. After expression of the transfected gene was established, selective conditions were chosen for the outgrowth of subclones that had undergone D-J joining and thus IgH enhancer deletion. Measurements of gamma 2a expression before and after enhancer deletion revealed that the enhancer was required for maintenance of expression of the transfected gene. The implication of this finding for models of enhancer function in endogenous genes is discussed.

Interethnic genetic differentiation: GM polymorphism in eastern Senegal

Analysis of GM polymorphism has been performed on 1,806 individuals representing three sympatric ethnic groups--Bedik, Fulani, and Mandenkalu--of eastern Senegal. Haplotype frequencies estimated by maximum likelihood have been used to compute common genetic pools between the three samples and a number of other sub-Saharan African populations. Despite extreme linguistic and sociocultural differentiations and very high levels of endogamy, especially in the Bedik and Niokholo Mandenkalu, the three populations share about 90%-95% of their haplotype frequencies in a system which commonly provides strong genetic differentiations. This supports the view that, despite its importance at a large continental scale level, as it is discussed for a set of populations from many regions of sub-Saharan Africa, sociocultural differentiation usually has little effect on local genetic diversity.

Left-handed Z-DNA and intramolecular triplex formation at the site of an unequal sister chromatid exchange

An unequal sister chromatid exchange (USCE) in the mouse myeloma cell line MPC-11 between 3' regions of the C gamma 2a and C gamma 2b heavy chain genes results in duplication of the C gamma 2a heavy chain gene and generation of a novel recombination joint. The USCE occurs between (TC)n tracts adjacent to alternating purine-pyrimidine tracts. We have investigated the capacity of both the donor regions and the recombinant product involved in this event to adopt left-handed Z-DNA and intramolecular triplexes. The results of chemical probing with diethylpyrocarbonate and osmium tetroxide at the base pair level demonstrate that under the influence of negative supercoiling the alternating purine-pyrimidine regions of these plasmids can adopt Z-DNA at neutral pH, and the oligopurine.oligopyrimidine (pur.pyr) regions of these regions can adopt intramolecular triplexes at low pH (less than or equal to pH 6.0). At intermediate pH values, mixtures of both structures are present. Increasing the negative superhelical density of the plasmid does not increase the amount of triplex present at neutral pH indicating that the presence of long Z-DNA segments adjacent to pur.pyr tract prevents intramolecular triplex formation. In summary, we conclude that the sequences involved in the USCE can form either an intramolecular triplex in the (TC)n tract or Z-DNA in the alternating purine-pyrimidine tract and that Z-DNA will predominate under physiological conditions. The presence of segments which adopt Z-DNA at a site of USCE suggests that formation of this structure may enhance recombination between adjacent pur.pyr tracts.

Delay of early B-lymphocyte development by gamma 2b immunoglobulin transgene: effect on differentiation-specific molecules

Mice transgenic for gamma 2b Ig heavy chain were examined for alterations in B-cell differentiation and endogenous Ig gene rearrangement and expression. Fresh bone marrow from these mice was markedly reduced in BP-1+ cells and there were small reductions in B220+ and sIg+ cells. A-MuLV (Abelson murine leukemia virus) transformants from these bone marrow cells showed little alteration in Ig gene rearrangement and expression when compared to controls, however. Isolation of the B-lymphoid compartment from these mice in vitro using LBMC (lymphoid bone marrow cultures) enabled more detailed characterization of the effects of the transgene. LBMC derived from gamma 2b transgenic mice had similar growth kinetics, but a 4-5-week delay in the expression of endogenous mu Ig in comparison to control cultures. Nucleic acids derived from these early cultures prior to endogenous mu Ig expression showed reduced Ig JH rearrangements, some sterile mu transcription, low levels of BP-1 expression, and virtually undetectable TdT (terminal deoxynucleotidyl transferase) expression. Thus, this gamma 2b transgene appears able to affect early B-lymphocyte development.

Structure of germline immunoglobulin heavy-chain gamma 1 transcripts in interleukin 4 treated mouse spleen cells

Antibody class switching is mediated by a DNA recombination event that replaces the C mu gene with one of the other heavy (H) chain constant region (CH) genes located 3' to the C mu gene. The regulation of this process is essential to the immune response because different CH regions provide different biological functions. Correlative evidence indicates that the isotype (class) specificity of the switch is determined by the accessibility of specific CH genes as indicated by hypomethylation and transcriptional activity. For example, RNAs transcribed from specific unrearranged CH genes are induced prior to switching under conditions that promote subsequent switching to these same CH genes. The function of transcription of these germline CH genes is unknown. In this report, we describe the structure of RNA transcribed from unrearranged gamma 1 genes in mouse spleen cells treated with LPS plus a HeLa cell supernatant containing recombinant interleukin 4. The germline gamma 1 RNA is initiated at multiple start sites 5' to the tandem repeats of the gamma 1 switch (S gamma 1) region. As is true for analogous RNAs transcribed from unrearranged gamma 2b and alpha genes, the germline gamma 1 RNA has an I exon transcribed from the region 5' to S gamma 1 sequences, which is spliced at a unique site to the C gamma gene. The germline gamma 1 RNA has an open-reading frame (ORF) that potentially encodes a small protein 48 amino acid in length.

Regulation of IgG production by suppressor Fc gamma RII+ T hybridomas

In this work, we analyzed the immunoglobulin heavy (H) and light (L) chain production by two variant B hybridomas, UN2.C3 and UN2.C17.K1 co-cultured with cells from a Fc gamma RII+, IgG-binding factor (IgG-BF)-producer T hybridoma (T2D4.C1) or with cells of a Fc gamma RII-, IgG-BF-nonproducer variant (D10C5). We showed that only the Fc gamma RII+ hybridoma directly inhibits the IgG secretion by UN2.C3 through a soluble mediator. This inhibition affects the H and L chain synthesis as well as the H and L chain-encoding mRNA steady state. No apparent cytotoxic effect could be detected. In contrast, the production of kappa chain by an H chain-negative variant (UN2.C17.K1) was unaffected. This indicates that a complete IgG molecule is required to observe the inhibitory effect induced by T2D4.C1. The pattern of effector/target cell interactions observed in our work suggests that the soluble factor involved in the suppression of IgG production is IgG-BF, able to transiently modify the IgG gene expression in target cells.

Structure and expression of germline immunoglobulin gamma 3 heavy chain gene transcripts: implications for mitogen and lymphokine directed class-switching

We have characterized the structure and expression of transcripts synthesized from the murine germline immunoglobulin gamma 3 heavy chain gene in certain B-lineage cells. The transcripts initiate upstream of the switch gamma 3 region, generating a 5' exon that is spliced to C gamma 3 exons. Expression of this germline transcript is induced when splenic B cells or A-MuLV-transformed pre-B cell lines are cultured in the presence of lipopolysaccharide (LPS). Addition of interleukin-4 (IL-4) to these lipopolysaccharide (LPS) cultures dramatically inhibits induction of the germline gamma 3 transcript. Induction of germline gamma 3 transcripts occurs before the increased accumulation of gamma 3-producing cells and VDJ-gamma 3 mRNA in cultures of splenic B cells. These data provide further evidence that germline CH transcriptional units are important components in the regulation of heavy chain class-switching. In addition, the pre-B cell lines that we describe represent the first example of permanent cell lines that regulate expression of the germline gamma locus in response to LPS plus IL-4 treatment in a manner analogous to normal B cells; therefore these lines should represent an excellent model system to further study the molecular mechanisms by which germline expression is regulated by these agents.

High-level expression of chimeric antibodies using adapted cDNA variable region cassettes

A rapid and generally applicable method for the modification of immunoglobulin cDNAs was developed so that the variable (V) regions could be expressed as cassettes, together with a variety of constant regions. Murine cDNAs were isolated, sequenced and the V regions joined to short oligonucleotides providing both splice donor sites and unique restriction sites for insertion into an expression vector. Using this strategy we have expressed the V regions of several murine antibodies, together with the human gamma 1 constant region. Although most of these chimeric antibodies were readily expressed, one murine light-chain cDNA sequence could not be expressed in transfected hybridoma cells. Reconstruction experiments indicate that the sequence created by the fusion of the murine leader and variable region blocked expression at the level of RNA accumulation. The methods described, as well as the potential problems of expression, are applicable to both traditional cDNA fragments and those obtained by in vitro amplification techniques.

Production of antibodies in transgenic plants

Complementary DNAs derived from a mouse hybridoma messenger RNA were used to transform tobacco leaf segments followed by regeneration of mature plants. Plants expressing single gamma or kappa immunoglobulin chains were crossed to yield progeny in which both chains were expressed simultaneously. A functional antibody accumulated to 1.3% of total leaf protein in plants expressing full-length cDNAs containing leader sequences. Specific binding of the antigen recognized by these antibodies was similar to the hybridoma-derived antibody. Transformants having gamma- or kappa-chain cDNAs without leader sequences gave poor expression of the proteins. The increased abundance of both gamma- and kappa-chains in transformants expressing assembled gamma-kappa complexes was not reflected in increased mRNA levels. The results demonstrate that production of immunoglobulins and assembly of functional antibodies occurs very efficiently in tobacco. Assembly of subunits by sexual cross might be a generally applicable method for expression of heterologous multimers in plants.

Further evidence that BALB/c and C57BL/6 gamma 2a genes originate from two distinct isotypes

Gene conversion by the corresponding gamma 2b gene has been proposed to explain the multiple differences between the nucleic acid sequences of BALB/c (Igh-1a) and C57BL/6 (Igh-1b) gamma 2a immunoglobulin allelic genes. However, genetic analysis indicates that duplicated forms of gamma 2a genes are not only present in Eastern Asia, but also in European wild mouse populations which suggests a widespread phenomenon. In order to verify whether the gamma 2a-related isotypic genes, namely gamma 2c and gamma 2a, could correspond to those present as alleles in domestic mice (Igh-1b and Igh-1a), a genomic library from Mus m.musculus strain (MAI) was constructed. Extensive mapping of the recombinant phages and Southern blot analysis with several restriction enzymes gave the complete organization of these loci: gamma 2b (18 kb) gamma 2c (17 kb) gamma 2a (14 kb) epsilon. The homology in flanking, coding and intervening region sequences indicates that MAI gamma 2c and gamma 2a related genes correspond to C57BL/6 and BALB/c Igh-1 alleles respectively. Also, Southern blot analysis using several probes derived from exonic and intronic regions between gamma 2b and gamma 2a genes shows a 2.0- to 3.0-kb difference in the distance between gamma 2b and gamma 2a genes of BALB/c strain as compared to C57BL/6. Taken together, these results indicate that BALB/c and C57BL/6 gamma 2a genes could originate from different isotypes.

Genetic analysis of new restriction fragment length polymorphisms (RFLP) in the human IgH constant gene locus

The human immunoglobulin heavy chain constant gene locus (IGHC) is polymorphic at both the protein (Gm and A2m allotypes) and the DNA level [RFLP for the gamma genes (IGHG), the switch mu region (IGHSM) and the switch alpha regions (IGHSA)]. The polymorphisms have been a valuable tool for assessment of the IGHC locus organization and a variety of population genetics and immunological investigations. In this study three new probes, identifying regions related to the IGHG (IGHPG and IGHSG) or IGHA (IGHAT) genes, have been employed to describe 11 different loci, 6 of which were polymorphic. Most of the polymorphisms are probably due to short insertions/deletions, particularly the SG regions, due to their repetitive structure. Ten loci were assigned to the IGHC region on the basis of known restriction maps, deletion mapping and association with mapped RFLP; the 11th, despite a striking sequence similarity with the IGHPG regions, could not be assigned to any known IGHC subregion. Analysis of these and previously known IGHG RFLP in a sample of 65 unrelated subjects plus 15 families allowed us to draw a genetic map, with particularly high resolution in the GP-G2-G4 genes region, revealing a marked discontinuity in the linkage disequilibrium values between pairs of adjacent loci.