Decreased metastatic spread in mice homozygous for a null allele of the cystatin C protease inhibitor gene

AIMS

Increased or altered activities of cysteine proteases have been implicated in serious human disorders such as cancer, rheumatoid arthritis, sepsis, and osteoporosis. To improve the current knowledge of the regulatory role of a major mammalian cysteine protease inhibitor, cystatin C, in such disease processes, a cystatin C deficient mouse was generated and characterized.

METHODS

The mouse cystatin C gene was inactivated by insertion of a bacterial neo gene through homologous recombination in 129/Sv embryonic stem cells. Embryonic stem cell clones were injected into C57BL/6J blastocysts followed by injection of the blastocysts into pseudopregnant female mice. F1 offspring with agouti coat colour after mating of chimaeric males with C57BL/6J females were examined by DNA analysis, and mice carrying the targeted mutation were intercrossed to obtain homozygous cystatin C deficient (CysC-/-) mice. To study the role of cysteine proteases and their inhibitors in metastasis, the spread of B16-F10 melanoma cells in CysC-/- and wild-type mice was compared. Analysis of the formation of remote metastases was carried out by intravenous injection of beta-galactosidase transfected B16-F10 cells and subsequent determination of cancer cell colonies in the lungs.

RESULTS

Cystatin C deficient mice were fertile and showed no gross pathological abnormality up to 6 months of age. Compared with wild-type mice, seven times fewer large metastatic colonies were counted by means of a dissecting microscope in CysC-/- mice two weeks after tail vein injection of B16-F10 cells. At all of eight time points from 15 minutes to two weeks after intravenous injection of tumour cells, the CysC-/- mice had significantly fewer lung metastases. The observed differences were smaller when beta-galactosidase transfected cells were used to allow counting of small colonies. Subcutaneous and intracerebral tumour growth was not different in the CysC-/- mice.

CONCLUSIONS

Cystatin C concentrations in vivo might influence metastasis in some tissues. The decreased metastatic spread of B16-F10 cells in CysC-/- mice is the result of both reduced seeding and reduced growth of tumour cells in their lungs.

Clinical and histopathological characteristics in relation to aetiological risk factors in cutaneous melanoma: a population-based study

In this population-based, case-control study from Sweden using data collected from 1988 to 1990, an increased risk of melanoma was associated with the number of sunburns, propensity to freckle, the number of raised naevi and a family history of melanoma. Furthermore, a decreased risk was associated with occupational sun exposure. The purpose of this study was to investigate whether different histopathological features of the melanoma and clinical factors were related to the different aetiological risk factor patterns. All the confirmed primary cutaneous melanomas (n = 366) were included in the study. Both univariate analyses with tests for interaction and multivariate analyses were performed. Patients with melanoma on the trunk and patients with thin melanomas had an excess of close relatives with a history of melanoma (odds ratios [ORs] = 2.7 and 2.3, respectively). A relationship was also seen between melanomas in younger persons and a family history of melanoma (OR = 2.6). The presence of raised naevi on the arm had a tendency to be closer related to melanoma of the nodular type (OR = 4.3) than melanoma of the superficial spreading type (OR = 1.6). Patients with outdoor occupations during summer had a decreased risk of developing melanoma on the extremities. Melanoma diagnosed in patients born before 1939 had an association with sunburns (OR = 1.9) and freckling (OR = 2.0), while melanomas in patients born in 1939 or later were related to a family history of melanoma (OR = 2.2). These results suggest that different histopathological and clinical features of melanoma are associated with different risk factor patterns, which may imply diverging tumour genesis.

Estrogen receptor content in adenovirus type 9-induced rat mammary tumors

Hormonal factors play an important role in the induction of mammary tumors and tumor-like lesions in adenovirus type 9-inoculated W/Fu rats. Primary Ad 9-induced fibroadenomas contained significantly higher amounts of estrogen receptor (determined by means of enzyme immunoassay) in comparison to normal breast tissue (p = 0.01**) and "spontaneous" fibroadenomas (p = 0.03*), used as control tissues. The receptor content of serially isografted virus-induced fibroadenomas did not differ significantly from the two types of control tissue. The findings suggest that changes in the estrogen receptor level are of importance in the tumor induction process, but also that additional factors are required for the preservation of tumor characteristics as well as for lipoma induction.

Nonrandom numerical chromosome abnormalities in basal cell carcinomas

Clonal chromosome abnormalities were found in 22 of 23 short-term cultured basal cell carcinomas (BCC) of the skin. The karyotypic abnormalities were nonrandom and in several cases included evidence of clonal evolution. Especially in cultures showing an epithelial growth pattern, simple numerical changes, most commonly +18, +9, +20, +7, and +5, predominated and presumably constitute pathogenetically important aberrations present in the neoplastic parenchyma. Also, several structural rearrangements of chromosome arm 9q were seen, which may be of particular interest against the background that a gene for familial BCC (Gorlin syndrome), the PTCH gene, maps to this region. Finally, most of the clonal aberrations detected in predominantly fibroblast-like cultures are likely to reflect changes acquired by cells of the tumor stroma, which raises the question whether mutations also of this tumor component may play a pathogenetic role in BCC development.

Characterization of a malignant eccrine poroma by cytogenetic and fluorescence in situ hybridization techniques

We have cytogenetically and by fluorescence in situ hybridization (FISH) analyzed short-term cultures from a malignant eccrine poroma, a type of tumor in which chromosomal abnormalities have previously not been reported. Combining the results from G-banding and FISH, the karyotype was interpreted as 50,XX,t(3;11;12)(q21;p15;q15),+5,+7,+9, +9,t(12;22)(q13;q13),+del(12)(q15),der(15)t(3;15) (q23;p13),der(17)t(8;17)(q22;p13),-20,add(22)(q13).

Oral manifestations in 100 Zimbabwean HIV/AIDS patients referred to a specialist centre

OBJECTIVE

To document the prevalence, age, sex ratio and clinical features of oral lesions associated with HIV/AIDS among patients referred to the two largest specialist centres in Zimbabwe.

DESIGN

Prospective study.

SETTING

University Hospital.

SUBJECTS

100 consecutive patients with oral manifestations of HIV/AIDS were interviewed and examined by Dental Surgeons and Physicians.

METHODS

Inspection and palpation of head and neck was done in all cases. Examination of the oral cavity and oropharynx with torchlight and wooden spatula was performed and lesions recorded according to WHO recording form for oral lesions possibly related to HIV infection.

RESULTS

The median age was 35 years (range 19 to 66 years). The male to female ratio was 4:1. General symptoms and signs found were: weight loss in 52, diarrhoea in 34, lymphadenopathy in 21 and Herpes zoster in 12 patients. Ninety two patients had oral lesions of some sort. These consisted of: oral ulcerations in 26, candidiasis in 22, osteomyelitis in three, necrotizing gingivitis in two and herpetic gingivostomatitis in one patient. Neoplasms recorded were: Kaposi's sarcoma in 72 patients, squamous cell carcinoma in two patients. Four other tumours not associated with HIV/AIDS were found. No hairy leukoplakia was found in this study.

CONCLUSIONS

The peak prevalence was in the age group 31 to 40 years with a male dominance of 4:1. The most frequent lesion was Kaposi's sarcoma, M:F ratio 3:1, followed by oral ulcers and candidiasis.

The reciprocal translocation t(9;16)(q22;p13) is a primary chromosome abnormality in basal cell carcinomas

The reciprocal translocation t(9;16)(q22;p13) was identified in three short-term cultured basal cell carcinomas (BCCs). The t(9;16) was the sole anomaly in one clone in two tumors and was accompanied by a second change that also affected the long arm of chromosome 9 in the third. In addition, other cytogenetically unrelated abnormal clones were also found in all three BCCs. The identification of t(9;16)(q22;p13) as a primary chromosomal abnormality in a subset of BCCs (we found it in 3 of 22 tumors) is especially intriguing against the background that the PTCH gene, which when mutated in the germ line presumably gives rise to the autosomal dominant basal cell nevus or Gorlin's syndrome, maps to chromosome band 9q22. None of the genes rearranged in the BCC-specific t(9;16)(q22;p13) translocation have been identified, but we hypothesize that the translocation represents the cytogenetic corollary of a tumorigenic recombination of PTCH with an as yet unknown gene in 16p13. If so, this would be the first time that a tumor suppressor gene causally involved in a hereditary cancer is shown to be frequently rearranged through a specific translocation in sporadic carcinomas of the same type.

Cutaneous malignant melanoma in southern Sweden 1965, 1975, and 1985. Prognostic factors and histologic correlations

BACKGROUND

There is a worldwide increase in the incidence of cutaneous malignant melanoma (CMM) among whites. In Sweden, a five-fold increase has been recorded since 1960, although the increase in mortality rate is substantially lower. Tumor thickness is recognized as the most important histologic prognostic factor for primary melanoma. In a previous study, the authors did not find any significant decrease in mean tumor thickness over the period 1965-1985 in their region. In the current study, prognostic factors for melanoma were evaluated for this time period.

METHODS

In a population-based study, 468 cases of invasive melanoma, diagnosed in the years 1965, 1975, and 1985, were histopathologically reexamined. The level of invasion, tumor thickness, regressive reaction, ulceration, presence of inflammatory cells, presence of benign nevus cells, and site of presentation were studied. In 461 of these 468 patients, it was possible to correlate the histopathologic factors with survival.

RESULTS

In univariate analyses, the parameters of presence of ulceration, increasing tumor thickness, male gender, nodular type of melanoma, and older age at diagnosis were significantly related to a shortened overall survival. In various multivariate models with adjustment for age and the factors studied simultaneously, ulceration, increasing tumor thickness, and male gender were significantly associated with a poor prognosis. Correlations between the factors studied were noted. It was observed that older patients tended to have thicker tumors. Thick melanomas correlated to a deeper level of invasion (Clark's), nodular growth pattern, ulceration, less inflammation, and less regression compared with thin, less invasive melanomas. Women had significantly fewer inflammatory cells and fewer signs of regression in their tumors compared with men.

CONCLUSIONS

In multivariate analyses adjusted for age, increasing tumor thickness, older age, ulceration, and male gender were significantly associated with a poor prognosis among patients with invasive CMM. None of these factors showed a significant change for the years 1965, 1975, and 1985. Thus, a change in the prognostic factors studied does not explain the increased survival of melanoma patients for this time period.

Carbonic anhydrase II and carbonic anhydrase-related protein in the cerebellar cortex of normal and lurcher mice

The developmental profiles of carbonic anhydrase II (CA-II) and a carbonic anhydrase related protein (CARP) were studied in rat and mouse cerebella. Enzyme histochemistry, immunohistochemistry, in situ hybridisation and Western blotting were used to study the synthesis and expression of these enzymes in cerebellar sections from age matched control, CA-II deficient and lurcher mice, the latter being characterised by Purkinje cell degeneration. Both CA-II and CARP were first found to be expressed in the Purkinje cells in the 9 day old mouse, and the immunoreactivity of both peptides increased with time. Immunohistochemistry showed more intense staining of CARP than of CA-II in Purkinje cells throughout the developmental profile of the mouse, and this was mirrored by the mRNA levels determined by in situ hybridisation. Immunohistochemistry of CA-II and CARP also demonstrated the progressive dendritic growth of the mouse and rat Purkinje cells. CA-II and CARP immunoreactivity ceased by the end of cerebellar maturation. The onset of Purkinje cell degeneration was detected at day 10 in the lurcher mouse, with concomitant marked decrease in CA-II level: however CARP expression was found to be unchanged. By postnatal day 16 neither CA-II mRNA, protein, nor activity was detectable in contrast to CARP which remained at a decreased level unit the Purkinje cells population had completely degenerated. Our findings suggest a role of CA-II in the degenerative processes of the lurcher Purkinje cells, with CARP playing an important role in the development and maturation of the cerebellar cortex.

Risk of malignant melanoma in relation to drug intake, alcohol, smoking and hormonal factors

In a population-based, matched case-control study from southern Sweden of 400 patients with a first diagnosis of malignant melanoma and 640 healthy control subjects aged 15-75 years, the association between commonly prescribed drugs, alcohol, smoking and malignant melanoma was evaluated. In addition, the relation between reproductive and hormonal factors and melanoma in women was studied. It was found that certain specific types of prescribed drugs, i.e. beta-blockers, hydralazines and benzodiazepines, may increase the risk of melanoma development. However, these associations were diminished, at least for benzodiazepines, after controlling for host factors. As these findings are unconfirmed, and may be due to chance or confounding, further studies are warranted. The risk of malignant melanoma was not influenced by alcohol consumption or smoking habits. Our results do not suggest an association between oral contraceptives and melanoma. Furthermore, reproductive factors were not independent risk factors for melanoma. However, increasing number of live births seemed to be protective (P for trend = 0.01). There is a need for further research to be able to draw firm conclusions on the relation between number of live births and melanoma. The results based on histopathological re-examinations and those based on tumour registry data were essentially the same.

Influence of body temperature on thyrotropic hormone release and lipolysis in the newborn infant

The present study investigated possible interactions between body temperature, lipolysis and thyrotropin (TSH), the only hormone with a documented lipolytic effect in vitro in newborn infants. Healthy infants were either nursed in the usual way (n = 18) or protected from a decrease in body temperature (n = 17) during the first postnatal hour. The infants' axillary temperatures were measured immediately after birth and after 10 and 60 min. Blood samples were collected from the umbilical vein and from the infants 10 and 60 min after birth for analysis of TSH, glycerol, free fatty acids, 3-OH-butyric acid and glucose. We found that the mean (+/- SD) infant axillary temperature was 37.6 +/- 0.4 degrees C immediately after birth. In the routinely nursed infants, body temperature decreased to 37.0 +/- 0.5 degrees C at 10 min (p = 0.01) and to 36.6 +/- 0.4 degrees C at 60 min (p = 0.01); the cold-protected infants maintained their fetal temperature at 60 min of age. There was a four-fold increase in plasma TSH levels at 10 min, independent of the infant's body temperature, and the hormone level remained invariably high at 60 min. Plasma glycerol levels increased progressively at 10 min (p = 0.01) and 60 min (p = 0.01) in both infant groups, but were higher (p = 0.02) in the routinely nursed infants at 60 min. No significant relationship was found between TSH and glycerol levels. Infant body temperature did not affect the levels of free fatty acids, 3-OH-butyric acid or glucose. We conclude that the change in environmental temperature as a result of extrauterine adaptation causes thermal stimulation of the infant's body surface which leads to activation of the hypothalamic-pituitary TSH axis, resulting in maximal TSH release, and thus to induction of the lipolytic process. A decrease in body temperature may be an additive stimulus for further enhancement of lipolysis.

Is the use of sunscreens a risk factor for malignant melanoma?

The relation between use of sunscreens, different host factors and malignant melanoma was investigated in a population-based, matched case-control study of malignant melanoma in the South Swedish Health Care Region, which has the highest risk for melanoma in Sweden, between 1 July 1988 and 30 June 1990. In total, 400 melanoma patients and 640 healthy controls aged 15-75 years answered a comprehensive questionnaire regarding different epidemiologic variables, including questions on use of sunscreens and different constitutional factors. The use of sunscreens was not found to protect against developing malignant melanoma. Instead, an unexpected relation between the use of sunscreens and the risk of developing malignant melanoma was seen (odds ratio (OR) 1.8 for almost always vs never using sunscreens). A tentative dose-response relation was found. Virtually the same ORs were seen in both sexes. Furthermore, persons younger than 50 years had a higher OR than persons older than 50 years. When different melanoma presentation sites were considered, lesions of the trunk were associated with sunscreen use in females (adjusted OR = 3.7 for almost always vs never using sunscreens), while lesions of the extremity or head and neck were associated with sunscreen use in males (adjusted OR = 3.2 for almost always vs never using sunscreens). Raised naevi on the left arm and freckling were shown to be the major constitutional risk factors (OR = 3.9 for more than three naevi vs none and OR = 1.4, respectively). The results were essentially unaltered in a histopathologically re-examined material. Further investigations are needed in order to form a basis for melanoma prevention.

Use of sunbeds or sunlamps and malignant melanoma in southern Sweden

In a population-based, matched case-control study from the South Swedish Health Care Region, which has the highest risk for melanoma in Sweden, the relation between the use of sunbeds or sunlamps and malignant melanoma was investigated. Between July 1, 1988, and June 30, 1990, a total of 400 melanoma patients and 640 healthy controls aged 15-75 years answered a comprehensive questionnaire containing different epidemiologic variables. Questions regarding the use of sunbeds or sunlamps were included. The odds ratio for developing malignant melanoma after ever having used sunbeds or sunlamps was 1.3. Considering all age groups, the odds ratio was significantly elevated after exposure more than 10 times a year to sunbeds or sunlamps (odds ratio (OR) = 1.8). When the study was restricted to patients and controls younger than age 30 years because the use of tanning devices is much more common among young persons, the odds ratio was higher (OR = 7.7 for more than 10 times a year vs. none). These findings were independent of constitutional factors and factors regarding sun exposure. A dose-response relation was evident. Furthermore, among melanoma patients in this young age group, the ratio of females to males was significantly higher than in older patients. When different melanoma presentation sites were considered, only lesions of the trunk were significantly associated with sunbed or sunlamp use (OR = 4.2 for more than 10 times a year vs. none).

Cytogenetic heterogeneity in a clear cell hidradenoma of the skin

Short-term cultures from a clear cell hidradenoma, a benign skin tumor for which no chromosome data exist, were cytogenetically analyzed. A total of eight unrelated aberrant clones were identified. The karyotypic profiles of two separately processed parts of the sample--a tumor nodule and seemingly normal adjacent dermal tissue--were different. Characteristic for the tumor nodule was a single abnormal clonal population consisting of three subclones: 46,XY,der(2)inv(2)(p13q23)t(2;9)(p13;q22), der(9)t(2;9)(q23;q22),t(11;19)(q21;p13),t(12;19)(q24;p13)/46,idem, inv(1)(p32q44)/92,idemx2. The adjacent tissue contained, in addition to the clone found in the tumor nodule, a spectrum of unrelated clones, the largest of which also showed clonal evolution: 45-47,XY,t(3;6)(p25;p25),t(12;17)(q15;q12),-17,+r(17)x2 [cp]/45-47,idem,inv(5)(p15q22)/90-94,idemx2. The remaining six clones found in this part were small and had simpler numerical or structural aberrations. The multiclonal pattern observed in this hidradenoma seems to reflect both cytogenetic convergence and divergence during neoplastic progression. The presence of unrelated clones may be an indication that the tumor was of multicellular origin.

Cutaneous malignant melanoma in south Sweden 1965, 1975, and 1985. A histopathologic review

BACKGROUND

There is an increase in the incidence of cutaneous malignant melanoma (CMM) among white people throughout the world. In Sweden, a fivefold increase has been recorded since 1960, but the mortality is rising at a much lower rate. Tumor thickness is the single most important prognostic factor for primary melanoma. This study aimed to clarify whether the thickness of the tumor in invasive CMM decreased during the period 1965-1985.

METHODS

This population-based study identified 574 cases of CMM, both invasive and noninvasive, in the South Swedish Health Care Region in 1965, 1975, and 1985. Twenty-six cases were excluded because the collection or evaluation of the material was not possible. The remaining 548 cases were reviewed histopathologically, and a diagnosis of invasive CMM was rejected in 71 cases. Eventually, 467 cases of invasive melanoma remained in our study (70 in 1965, 124 in 1975, and 273 in 1985). The level of invasion, tumor thickness, regression, ulceration, presence of inflammatory cells, benign naevus cells, and the site of presentation were studied.

RESULTS

The study found neither a significant decrease of tumor thickness of invasive CMM nor changes in the level of invasion or proportion of ulcerated melanoma. A significantly higher proportion of melanoma tumors containing benign naevus cells was seen throughout the years (P < 0.05). Women had significantly fewer inflammatory cells in their tumors than did men (P < 0.01). As expected, the anatomical site of presentation showed a significant sex-related difference, with more tumors on the legs of female patients and more on the trunk of male patients (P < 0.001).

CONCLUSIONS

There is a divergence between the rapidly increasing incidence and the slower increase in mortality of CMM. This cannot be explained by a removal of the melanoma at a thinner thickness. Differences between the sexes according to the tumor site and the increasing rate of CMM containing benign naevus cells could implicate changes in the tumor biology. Public education in Sweden concerning ultraviolet radiation and the connection with melanoma is fairly new and might not have any influence on this time period. Additional investigation is needed to clarify this matter.

Clonal structural chromosome aberrations in nonneoplastic cells of the skin and upper aerodigestive tract

Cytogenetic analyses of tumors of the skin and upper aerodigestive tract have repeatedly revealed small, pseudodiploid clones characterized by balanced structural rearrangements and a high frequency of cells with nonclonal structural aberrations. However, the lack of common cytogenetic denominators within the different histologic subtypes, the discrepancy between cytogenetic findings and data obtained from flow cytometric DNA content studies, and the occasional identification of tumors with massively rearranged karyotypes indicate that the chromosome rearrangements present in pseudodiploid cells have little to do with the tumorigenesis or progression. Further support for this conclusion, and indirect evidence that the pseudolipid clones probably do not represent the tumor cell populations, derives from the present study in which clonal and nonclonal structural rearrangements were also found in short-term cultures from nonneoplastic skin and pharyngeal mucosa. It is possible that the aberrations are present in subepithelial fibroblast that have accumulated DNA damage due to extensive exposure to potentially carcinogenic agents.

Cytogenetic analysis of 33 basal cell carcinomas

Cytogenetic analysis of short-term cultures from 33 basal cell carcinomas (BCC), a type of neoplasm for which no previous karyological data exist, revealed clonal chromosome aberrations, all of them different, in 8 tumors. In 2 cases, 2 cytogenetically unrelated clones were detected, suggesting a multicellular origin in at least a subset of BCC. A remarkably high level of nonclonal structural rearrangements, mostly in the form of seemingly balanced translocations, was found in 23 tumors; namely, in 6 of 8 BCC with clonal karyotypic abnormalities and in 17 of 25 without. It is possible that some of these aberrations represent additional neoplastic clones, thus indicating an even higher level of cytogenetic heterogeneity in BCC. We think that the most likely interpretation of the results is that BCC may have a multicellular origin, reflecting field cancerization of the skin. During subsequent tumor development, the selection pressure narrows down the number of clones that infiltrate the surrounding tissue. The finding by karyotypic analysis of some apparently monoclonal, some polyclonal BCC, may reflect that different tumors have been examined at different points in the clonal evolution of the neoplastic cells.

Clonal chromosome aberrations in a keratoacanthoma and a basal cell papilloma

Clonal chromosome abnormalities were found in short-term cultures from two epithelial skin tumors, a basal cell papilloma and a keratoacanthoma. The three-way translocation t(2;6;11)(q21;q27;p13) was the sole clonal rearrangement in the basal cell papilloma. The karyotype of the keratoacanthoma was more complex: 46,XX,der(2)(2pter----2p13::2p11----cen----2q37: :5q33----5qter),der(2) (:2p13----cen----2q37::6q23----6qter),der(5)t(2; 7;5)(q37;q11;q33),der(6) (6pter----cen----6q23::2p13----2pter),der(7)t(2; 7;5)(q37;q11;q33), del(13)(q11q14). In addition, several nonclonal structural changes were seen in both tumors.

Diverse chromosome abnormalities in squamous cell carcinomas of the skin

Short-term cultures from three invasive squamous cell carcinomas of the skin were cytogenetically analyzed. Clonal chromosome aberrations were found in all tumors. In the first case, two of three abnormal clones were related, and in the second case, two of five clones demonstrated cytogenetic similarities. Both clones detected in case 3 had a structural rearrangement in common. Several nonclonal changes were seen in all three cases in addition to the clonal aberrations. None of the rearrangements detected, clonal or nonclonal, corresponds to any of the consistently cancer-associated aberrations known from other neoplasms. The remarkably diverse karyotypic picture of the three squamous cell carcinomas, in particular the finding of unrelated clones in two of them, hints that these neoplasms may be poly-rather than monoclonal. The lack of a common cytogenetic denominator argues that if chromosomal changes are of pathogenetic importance in this tumor type, a wide variety of apparently dissimilar changes exist that are roughly equal in their capacity to malignantly transform skin epithelium.

Adenovirus type 9-induced tumorigenesis in the rat mammary gland related to sex hormonal state

Adenovirus type 9 was inoculated sc into newborn Wistar/Furth rats, divided into four groups: (1) six male rats, not treated further; (2) 11 male rats, castrated at 4 weeks of age; (3) 12 male rats, castrated at 4 weeks of age and subsequently treated repeatedly with estrogen; and (4) 12 female rats, not treated further. All of the rats in group 3 developed mammary hyperplasia and tumors (fibroadenomas and lipomas), in some cases with malignant histologic structure. Rats in group 4 developed similar mammary tumors, but with later appearance and significantly slower growth. A fifth group of rats, not virus inoculated but castrated and estrogen treated as in group 3, did not develop any demonstrable mammary lesions. The results show that the effects of the virus on the mammary gland are dependent upon an estrogenic background, which by itself cannot cause tumor development in males. It is suggested that viral DNA is incorporated into the cellular DNA in such a way that it influences the synthesis and/or activity of steroid receptors, triggering tumor development.