Murine mammary tumor virus expression during mammary tumorigenesis in BALB/c mice

A new infectious mammary tumor virus in the milk of mice implanted with C4 hyperplastic alveolar nodules

We have previously characterized an infectious mouse mammary tumor virus [(MMTV(SW)] which induces a strong superantigen response in vivo. Here we describe the isolation and characterization of MMTV(C4) which was derived from milk of mice implanted with hyperplastic alveolar nodules. MMTV(C4) stimulates V beta 2 expressing T cells after local injection in vivo. Comparison with known open reading frame (orf) sequences revealed high homology to Mtv-6, an endogenous virus interacting with V beta 3-expressing T cells. The carboxyl-terminal amino acids were, however, altered. High homology including the carboxyl-terminal orf amino acids were found with MMTV(C3H-K). We show here that MMTV(C3H-K) has lost its superantigen function. Sequence comparisons permitted the characterization of few key amino acids which could be important for T cell receptor interaction and superantigen processing.

An exogenous mouse mammary tumor virus with properties of Mls-1a (Mtv-7)

The classical minor lymphocyte stimulating (Mls) antigens, which induce a strong primary T cell response in vitro, are closely linked to endogenous copies of mouse mammary tumor viruses (MMTV). Expression of Mls genes leads to clonal deletion of T cell subsets expressing specific T cell receptor (TCR) V beta chains. We describe the isolation and characterization of a new exogenous (infectious) MMTV with biological properties similar to the Mls antigen Mls-1a. In vivo administration of either Mls-1a-expressing B cells or the infectious MMTV (SW) led to an increase of T cells expressing V beta 6 followed by their deletion. Surprisingly, different kinetics of deletion were observed with the exogenous virus depending upon the route of infection. Infection through the mucosa led to a slow deletion of V beta 6+ T cells, whereas deletion was rapid after subcutaneous infection. Sequence analysis of the open reading frames in the 3' long terminal repeat of both this exogenous MMTV (SW) and of Mtv-7 (which is closely linked to Mls-1a) revealed striking similarities, particularly in the COOH terminus, which has been implicated in TCR V beta recognition. The identification of an infectious MMTV with the properties of a strong Mls antigen provides a new, powerful tool to study immunity and tolerance in vivo.

Spontaneous progression of hyperplastic outgrowths of the D1 lineage to mammary tumors: expression of mouse mammary tumor virus and cellular proto-oncogenes

Mammary cancer in mice is characterized by progression through defined stages of preneoplasia, with the most common preneoplastic stage being the hyperplastic alveolar nodule (HAN). We determined the relative levels of RNA expression of various cellular proto-oncogenes and endogenous mouse mammary tumor virus genes in outgrowths and tumors of three sublines of the transplantable D1 HAN preneoplastic outgrowth line. The three sublines differed in relative tumor-producing capabilities. Subline D1B produced a high incidence of tumors with short latency periods, whereas sublines D1C and D1D produced low incidences of tumors with long latency periods. No consistent alteration in proto-oncogene expression correlated with relative tumorigenicity, although tumors frequently contained higher levels of one or more proto-oncogene transcripts as compared with preneoplastic tissue. Slightly elevated (2- to 6-fold) levels of different oncogene transcripts were detected in 13 of 17 tumors as compared with outgrowth tissue, including abl (2 tumors), fps (5 tumors), Ha-ras (6 tumors), and Ki-ras (8 tumors). One tumor contained 45 times more Ki-ras-specific RNA than outgrowth tissue because of a comparable amplification of Ki-ras DNA sequences. Elevated levels of Ha-ras occurred more frequently in tumors of a high-incidence subline than in a less-aggressive subline (5/10 vs 1/7), but this difference was not statistically significant. However, consistent changes in MMTV expression accompanied progression from preneoplastic tissues to mammary tumors. All 17 tumors displayed reduced levels of the MMTV-specific long terminal repeat (LTR) transcript (1.6 kb) as compared with HAN tissue; tumors with moderate levels of LTR transcript expressed the 3.8-kb envelope message as well, one not detected in HANs. Expression of the LTR transcript is apparently influenced by factors in addition to the methylation status of endogenous mouse mammary tumor virus genes, which was similar in outgrowths and tumors. As the survey of representative proto-oncogenes failed to identify a uniform change between HAN and tumors, it is likely that other genes are involved in tumor progression in the mammary gland.

Identification and characterization of a mouse mammary tumor virus protein uniquely expressed on the surface of BALB/cV mammary tumor cells

A unique subline of BALB/c mice, designated BALB/cV, exhibits an intermediate mammary tumor incidence (47%) and harbors a distinct milk-transmitted mouse mammary tumor virus (MMTV). The BALB/cV subline was used to study the molecular basis of potential virus-host interactions involving cell surface-expressed MMTV proteins. Cell surface iodination identified virus-specific proteins expressed on BALB/cV primary mammary tumor cells grown in culture. In contrast to (C3H)MMTV-producing cell lines which expressed MMTV gp52, BALB/cV tumor cells lacked gp52 and expressed instead a 68K, env-related protein. The 68Kenv protein was also detected on the surface of metabolically labeled BALB/cV tumor cells by an external immunoprecipitation technique. The expression of 68Kenv was restricted to mammary tissues of BALB/cV mice that also expressed other MMTV proteins. Biochemical analysis established that 68Kenv was not modified by N-linked glycosylation. 125I-labeled 68Kenv was rapidly released into the media of tumor cell cultures and was recovered both in the form of a soluble protein and in a 100,000 g pellet. The biologic function of this cell surface-expressed viral protein remains unknown.

Molecular basis of altered mouse mammary tumor virus expression in the D-2 hyperplastic alveolar nodule line of BALB/c mice

The preneoplastic D-2 hyperplastic outgrowth line, which was derived from a hormone-induced hyperplastic alveolar nodule (HAN) of a BALB/c mouse, was used for a detailed analysis of mouse mammary tumor virus (MMTV) expression. The D-2 HAN line has previously been shown to express viral RNA representative of the entire genome, although viral particles have been noted only rarely. The MMTV-specific mRNA, protein, and DNA content of the D-2 tissues was defined in an effort to better understand the molecular basis of the aberrant virus expression. Northern blotting techniques demonstrated the presence of properly processed 8.9 kb (genomic) and 3.6 kb (envelope) mRNA. Protein electroblotting procedures established the presence of properly processed viral core protein p28. In contrast, the envelope precursor polyprotein was not processed into detectable levels of gp52. Analysis of MMTV proviral content by Southern blot methodology revealed the presence of a newly acquired provirus which serves as a marker for the clonal nature of the D-2 line. The origin of the new provirus is unknown. Methylation studies established that the new proviral insert is hypomethylated and, therefore, is likely serving as the template for the MMTV expression observed in the D-2 HAN line. These characteristics of the D-2 line make it an excellent system in which to study the role, if any, of MMTV in the progression of D-2 preneoplastic tissues to the tumor phenotype.

Dysdifferentiative nature of aging: age-dependent expression of mouse mammary tumor virus and casein genes in brain and liver tissues of the C57BL/6J mouse strain

Many aspects of the aging process could be the result of cells slowly drifting away from their proper state of differentiation. This possibility has been studied by searching for an age-dependent increase in the expression of specific genes in tissues where expression of these genes would not normally be expected. In these studies, cDNA probes of specific genes are used in a DNA X NA hybridization assay to detect possible complementary RNA sequences in tissues of different-aged animals. Using this technique in past experiments, a qualitative increase in the RNA sequence complexity of mouse leukemia virus (MuLV) and a quantitative increase in the amount of alpha- and beta-globin RNA were found with increasing age in the brain and liver of the C57BL/6J mouse strain. We report here a similar age-dependent qualitative increase in the RNA sequence complexity for mouse mammary tumor virus (MMTV) but no quantitative or qualitative age-dependent change in casein RNA sequences for the same tissues and mouse strain.

Interaction of the heterozygous nude gene with the asplenia trait in mammary tumorigenesis

The BALB/c mouse strain has been shown to contain endogenous mouse mammary tumor virus (MMTV) proviral sequences. However, no exogenous MMTV particles have been detected in their tissues. Female BALB/c mice from our colonies exhibit a very low incidence of spontaneous mammary tumors (SMT); less than 1% at up to 20 mo of age. Immunodeficient BALB/c mice heterozygous for the nude gene (nu/+, +/+), for the dominant hemimelia gene associated with asplenia (+/+, Dh/+), or for both traits (nu/+, Dh/+) have been examined for SMT incidence and the presence of MMTV proviruses. Based on restriction digestion with Eco RI, Bam HI, and Pst I, the immunodeficient mice have an MMTV provirus copy number and organization identical to the BALB/cCrgl strain. This MMTV DNA pattern is distinct from the MMTV proviruses in C3H/He, C57BL/6J and CBA/CaJ mice, which were parental strains of the immunodeficient mutants. Normal female BALB/c or BALB/c heterozygous for the asplenic trait do not develop significant numbers of SMT at up to 19 mo of age. In contrast, an incidence of 23.8% and 57.7% SMT was observed in BALB/c nu/+ heterozygotes, and in BALB/c nu/+, Dh/+ heterozygotes, respectively. These results indicate that agenesis of the spleen, concomitant with the presence of the heterozygous nude gene, contribute to a high incidence of SMT in the low-SMT BALB/c mouse strain.

Selenium inhibition of the neoplastic transformation in preneoplastic mammary cell populations

The effect of dietary selenium on the tumor-producing capabilities of primary hyperplastic alveolar nodules (HAN) of the mammary gland of BALB/c mice was examined directly using a transplantation assay. The tumor-producing capabilities of primary HAN exposed to 2.0 ppm dietary selenium was decreased 57% compared to the tumor-producing capabilities of HAN exposed to 0.2 ppm dietary selenium. This degree of inhibition compared well with the 60% inhibition of mammary tumorigenesis when the mice were exposed to supplemental selenium during the entire tumorigenic period. The examination of primary hyperplastic alveolar nodules was a more accurate assessment of the responsiveness of the preneoplastic state than using established nodule outgrowth lines since the latter exhibited a greater degree of variability in response to selenium-mediated inhibition of growth and the mean response of numerous nodule outgrowth lines was not as great as seen in the primary HAN. These results indicate that selenium is an effective inhibitor of the neoplastic transformation in preneoplastic cells as well as the formation of preneoplastic cells.

Mouse mammary tumor virus DNA methylation: tissue-specific variation

Mouse mammary tumor virus-specific DNA sequences endogenous to the BALB/c mouse are shown to exhibit variable levels of methylation in a tissue-specific manner. In DNA from both lactating mammary gland and spleen, MMTV-specific sequences were hypomethylated at specific HpaII and HhaI sites. These variably methylated sites were found in the terminal repetitive sequences of the endogenous viral genomes. The specific hypomethylation of a HpaII site in Mtv-9 is associated with expression of a 1.6 kb transcript in the lactating mammary gland.

Stage specificity of selenium-mediated inhibition of mouse mammary tumorigenesis

The experiments reported herein examined the inhibitory role of selenium in chemical carcinogen-induced mouse mammary tumorigenesis. The results from four different experiments are presented herein and are summarized briefly. First, the results demonstrated that relatively low doses of dietary selenium (0.5-2.0 ppm) inhibited 7,12-dimethylbenzanthracene (DBMA)-induced mouse mammary tumorigenesis. At 2 ppm Se, the mammary tumor incidence was reduced from 56 to 15%. Second, the results suggested that the later stages of mammary tumorigenesis (preneoplastic to neoplastic transformation and tumor growth) are not as sensitive to selenium-mediated inhibition as the early stages, i.e., the induction and/or expression of mammary preneoplastic lesions. Finally, the results demonstrated that selenium markedly inhibited mammary tumorigenesis (from 42 to 8%) even when the mice were exposed to selenium only after the carcinogen treatments had been concluded. The results from these experiments are discussed from the viewpoint that selenium-mediated inhibition is a result of a direct block of DNA synthesis.

Transcription of mouse mammary tumor virus: identification of a candidate mRNA for the long terminal repeat gene product

We have examined an assortment of preneoplastic and neoplastic mouse mammary tissues for the presence of an mRNA which could encode the putative long terminal repeat gene product of mouse mammary tumor virus. We report here the detection of a novel mouse mammary tumor virus-specific, polyadenylic acid-containing transcript in certain preneoplastic and neoplastic mammary tissue of BALB/c mice. The molecule is 1.6 kilobases in length and contains sequences from the transcriptional leader and the U3 region of the proviral DNA. The upstream terminus of the 3' information lies 75 to 80 nucleotides from the beginning of the long terminal repeat open reading frame, in close proximity to a consensus splice acceptor in the DNA. The transcript was detected in hormonally or chemically induced neoplastic, preneoplastic, and lactating mammary tissue of BALB/c mice, but not in preneoplastic or tumor tissue induced by exogenous viruses in any strain of mice examined. This implies that the RNA we observed is transcribed from an endogenous provirus template.

Cloned mouse mammary tumor virus DNA exhibits glucocorticoid-dependent expression in simian virus 40-transformed mink cells

Mink lung epithelial cells were transfected with two cloned mouse mammary tumor virus (MMTV) DNAs, a 9-kilobase clone derived from an unintegrated exogenous viral genome and a 14-kilobase clone containing an integrated endogenous provirus along with cellular flanking sequences. Mink lung cells were chosen because they do not contain endogenous MMTV sequences. On the basis of our observation that simian virus 40 DNA efficiently transforms these cells, we isolated cell clones containing MMTV DNA by using transformation with simian virus 40 DNA as a selective marker in cotransfection experiments. Levels of the 9-kilobase MMTV mRNA representing the entire viral genome and of the spliced 4.4-kilobase mRNA which codes for the viral envelope proteins were glucocorticoid dependent in transformed cells. Expression of low levels of Pr77gag, the precursor of the group-specific viral core proteins, and of gPr73env, the precursor of the viral envelope proteins, was also hormone dependent. We conclude that these cloned MMTV DNAs contain all the information necessary for the synthesis of normal viral RNAs and proteins. These findings also provide further evidence that the DNA sequences involved in the hormone responsiveness of MMTV expression are contained within the viral genome.

Quantitative of murine mammary tumor virus-related RNA in mammary tissues of low- and high-mammary-tumor-incidence mouse strains

Lactating mammary glands and hormonally induced mammary tumors of BALB/c mice from three geographically separated breeding colonies were examined by molecular hybridization, using murine mammary tumor virus (MuMTV) cDNA representing the entire viral genome to determine the amount of MuMTV-related RNA expressed in these tissues. The RNA extracted from these tissues by the classical sodium dodecyl sulfate-pronase, phenol-chloroform procedure (method 1) contained barely detectable levels of MuMTV-related sequences. In contrast, both normal lactating mammary glands and hormonally induced mammary tumors of these mice were found to contain approximately one to two copies of the MuMTV genome per cell by using a new procedure in which the RNA was extracted with guanidine derivatives (method 2). No significant differences in the MuMTV-related RNA content of the BALB/c mammary tissues were observed regardless of their colony of origin. Our results suggest that expression of MuMTV RNA does not change in BALB/c mammary glands during transformation to a malignant state and that MuMTV expression does not play a role in tumorigenesis in these mice. In view of the increased recovery of MuMTV-related RNA from BALB/c mice with method 2, we compared the level of MuMTV RNA expression in lactating mammary glands and mammary tumors of other mouse strains, including C57BL/6 and RIII, using both extraction methods. Yields of MuMTV-related RNA from mammary tissues increased by as much as 35- to 40-fold, using method 2 as compared with method 1. Therefore method 2, involving guanidine derivatives, appears to be method of choice for MuMTV-related RNA extraction from the mammary tissues of certain strains of mice, particularly those expressing relatively low levels of MuMTV RNA.

Mammary tumor virus proviral DNA in normal murine tissue and non-virally induced mammary tumors

The Southern DNA filter transfer technique was used to study the involvement of the endogenous mouse mammary tumor virus (MMTV) in the development of mammary tumors of nonviral etiology. The presence of extra MMTV proviruses in the genomes of these non-virally induced mammary tumors would indicate an integration of the provirus of an activated endogenous MMTV. Acquisition of MMTV proviruses did not seem to be an absolute requirement for the development of hormone or carcinogenically induced mammary tumors in strain BALB/c nor for hormone-induced mammary tumors in mouse strains 020, C57BL, and C3Hf. In some hormone-induced mammary tumors we did observe extra MMTV proviruses in submolar quantities, indicating that reintegration may occasionally occur and that only a part of the tumor cells acquired new MMTV DNA information. Hormone-dependent and -independent primary mammary tumors of the mouse strain GR, which are controlled by the Mtv-2 mammary tumor induction gene, all acquired extra MMTV proviruses. Most of these extra MMTV proviral-DNA-containing fragments appeared present in submolar quantities, suggesting that only part of the tumor cells acquired extra MMTV proviral information. These findings indicate that the initially transformed mammary gland cells of non-virally induced mammary tumors do not necessarily acquire extra MMTV proviral DNA information, in contrast to the MMTV-induced mammary tumors, in which all tumor cells contain extra MMTV DNA information.

Partial expression of endogenous mouse mammary tumor virus in mammary tumors induced in BALB/c mice by chemical, hormonal, and physical agents

The possible interaction of environmental factors with the endogenous mouse mammary tumor virus (MMTV) genome in the development of mammary tumors in the low-tumor-incidence BALB/c mouse strain was examined. Tumors were induced in virgin female animals by treatment with chemical carcinogen 7,12- dimethylbenz[alpha]anthracene or urethan, with or without prolonged hormonal stimulation, or by X-irradiation. Concomitant hormonal stimulation resulted in increased tumor incidences compared with those induced by chemical carcinogen treatment alone. The frequency of tumor induction by irradiation alone or in combination with urethan or prolactin stimulation was very low. MMTV expression in the mammary tumors was assayed by nucleic acid hybridization and by immunohistochemical staining. Depending upon the treatment group, 0 to 89% of the tumors contained detectable levels of MMTV RNA (>/=0.0005% of the total cellular RNA). Tumors which contained detectable viral transcripts exhibited only low levels of MMTV RNA, which did not appear to represent the accumulation of RNA sequences homologous to the entire MMTV genome; synthesis of MMTV structural proteins was detected in only one tumor. Viral RNA-positive tumors were generally associated with a longer latent period. MMTV RNA expression occurred in tumors classified histologically as adenoacanthomas, as well as in mammary adenocarcinomas, although the cell types in the adenoacanthomas expressing viral RNA were not identified. It does not appear that expression of the endogenous MMTV genome is required for maintenance of all mammary tumors in BALB/c mice, although partial genome expression undetectable by the methods employed cannot be ruled out. Linear regression analyses were performed. The mean time to tumor appearance and the percentage of tumors which were MMTV RNA positive were found to vary linearly as a function of the total dose of 7,12-dimethylbenz[alpha]anthracene administered. The percentage of tumors which were MMTV RNA positive was also shown to be linearly related to the mean time to tumor appearance. These relationships provide a basis for predictions in the BALB/c system related to these parameters.

Mammary tumor induction loci in GR and DBAf mice contain one provirus of the mouse mammary tumor virus

The mammary tumor induction genes Mtv-1 in mouse strain DBAf and Mtv-2 in strain GR control the complete expression of the endogenous mouse mammary tumor virus (MMTV). We have used a combination of genetic, biochemical and molecular biological methods to identify and correlate specific copies of the endogenous MMTV proviral genes with the biological properties of the tumor induction genes Mtv-1 and Mtv-2. These Mtv induction genes contain specific MMTV proviral information, as was concluded from restriction enzyme analysis and molecular hybridization of DNAs of congenic mouse strains and of progenitors of backcross populations. The congenic strains differed from the parental strains GR and 020 only in the Mtv-2 gene, one lacking the Mtv-2 gene (GR/Mtv-2-) and one having obtained this gene (020/Mtv-2+). The gain or loss coincided with two Eco RI cellular DNA fragments containing MMTV DNA information. Since Eco RI cuts the exogenous proviral variant of MMTV DNA once, we assume that these two cellular DNA fragments contain one MMTV provirus. The same cellular DNA fragments containing MMTV DNA information segregated together with MMTV expression in the offspring population of the backcross. In a similar backcross analysis of the induction gene Mtv-1 it was also demonstrated that the Mtv-1 gene comprises one MMTV provirus. These data indicate that Mtv induction genes contain specific but different MMTV proviral genes and that nly a limited number of the MMTV proviruses present in the cellular DNA is associated with the control of proviral expression.

Quantification of mouse mammary tumor virus structural proteins in hormone-induced mammary tumors of low mammary tumor mouse strains

The expression of the mouse mammary tumor virus (MMTV) in hormone-induced mammary tumors was investigated by means of a radioimmunoassay for two major MMTV proteins, gp52 and p27. MMTV proteins were isolated on lectin affinity- and ion-exchange chromatography columns. The purified viral proteins were electrophoretically homogeneous and retained immunoreactivity after labelling with 125iodine. Standard competition assays showed that group-specific antigenic determinants were reacting. Mammary tumors were induced in three strains of mice with a low natural incidence of mammary tumors, C57BL, O20 and C3Hf, by a combined hormone treatment, consisting of hypophyseal isografts and administration of progesterone and estrone. Mammary tumors and mammary glands of hormone-treated animals were extracted and used for competition radioimmunoassays. In general, the tumorigenic hormone treatment resulted in enhanced amounts of MMTV proteins in the mammary glands, compared to the amounts found in lactating mammary glands of untreated animals. The levels of MMTV proteins in the mammary tumors were lower than in the mammary glands.

Selenium-mediated inhibition of mouse mammary tumorigenesis

The effects of selenium of mouse mammary tumorigenesis were examined in 2 systems. BALB/cfC3H (MuMTV-S positive) given 2 and 6 ppm selenium in the drinking water and maintained as breeding mice, had significantly lower incidences of mammary tumors (48% and 12%, respectively) than untreated control mice (82%). Selenium at 4 ppm inhibited the tumor-producing capabilities of only 1 of 4 preneoplastic outgrowth lines maintained in BALB/c (MuMTV-S negative) mice. Selenium at 2, 4 and 6 ppm had no effect on the growth rate of primary mammary tumors transplanted subcutaneously in BALB/c mice. Selenium inhibited mammary tumorigenesis without interference of normal reproductive function and weight gain.

Hormonal regulation of murine mammary tumor virus RNA expression during mammary tumorigenesis in BALB/c mice

The effects of glucocorticoids and prolactin on murine mammary tumor virus (MuMTV) RNA expression in preneoplastic outgrowth lines and mammary tumors in BALB/c mice were investigated. Hyperplastic alveolar nodules (HAN) and a ductal hyperplasia (DH) are induced in virgin BALB/c mice by prolonged hormonal stimulation or treatment with 7,12-dimethylbenz(a)anthracene or both. Mice bearing HAN or DH outgrowth lines and mammary tumors that arose from the outgrowth lines were treated with glucocorticoids or prolactin. MuMTV RNA was quantitated by hybridization with a representative complementary DNA probe specific for MuMTV RNA. Prolactin treatment did not increase MuMTV RNA in the BALB/c HAN or DH outgrowth lines or tumors. MuMTV RNA increased after glucocorticoid treatment in the C3, C4, and C5 HAN outgrowth lines and in tumors that arose from the D1, D2, C4, and C5 HAN and CD8 DH outgrowth lines. No increase in MuMTV RNA with glucocorticoid treatment was observed in the D1 or D2 HAN outgrowth line, in the CD8 DH outgrowth lines, and in tumors that arose from the C3 HAN outgrowth line. The ability of glucocorticoids to stimulate MuMTV expression was specific since the response was dose dependent and specific for glucocorticoid hormones. Glucocorticoid treatment did not increase the level of type C viral RNA in the majority of hormone- or 7,12-dimethylbenz(a)anthracene-induced HAN outgrowth lines or tumors. These observations suggested that glucocorticoids may influence MuMTV expression during mammary tumorigenesis in BALB/c mice.

Mouse mammary tumor virus genome expression in chemical carcinogen-induced mammary tumors in low- and high-tumor-incidence mouse strains

Involvement of mouse mammary tumor virus (MMTV) in 7,12-dimethylbenz[a]anthracene (DMBA)-induced mammary tumorigenesis was investigated in low- (BALB/c) and high- (BALB/cfC3H) mammary-tumor-incidence mouse strains. Both strains contain endogenous MMTV integrated into the cellular genome. Additionally, BALB/cfC3H mice are infected with exogenous MMTV-S which is responsible for a higher incidence of mammary tumors in breeding females. Administration of DMBA to virgin mice of both strains resulted in a moderate frequency of mammary tumors within 40 wk after treatment. No differences were found in DMBA-induced tumor incidences at 18 wk (6% and 7%) or at 38 wk (29% and 36%) after treatment of BALB/c and BALB/cfC3H mice, respectively. Expression of MMTV in these tumors was examined by assaying for the presence of MMTV RNA by hybridization using MMTV-specific cDNA and by immunohistochemical staining utilizing antibodies against MMTV 52,000-dalton glycoprotein, gp52, and 28,000-dalton internal protein, p28. Of 16 BALB/c tumors assayed, 11 did not contain detectable levels of MMTV RNA and the remaining 5 tumors contained only low levels (0.0005-0.0010%) of viral RNA. Importantly, MMTV RNA was not detected in 5 of 27 BALB/cfC3H tumors. The other BALB/cfC3H tumors contained quantities of MMTV RNA ranging from 0.0006 to 0.4170%. Most BALB/cfC3H tumors with detectable levels of MMTV RNA also synthesized viral proteins gp52 and p28. Thus, expression of the complete MMTV genome is not requisite for maintenance of the tumor phenotype in DMBA-induced mammary tumors in either BALB/c or BALB/cfC3H virgin mice under 1 year of age.