Analysis of the expression of spleen focus-forming virus (SFFV)-related RNA and gp55, a Friend and Rauscher virus-specific protein

The myeloproliferative sarcoma virus causes transformation or erythroid progenitor cells in vitro

The myeloproliferative sarcoma virus induces spleen focus formation in vivo and transforms fibroblasts in vitro. We showed in this study that in vitro infection of spleen or bone marrow cells from susceptible mice with the myeloproliferative sarcoma virus leads to the formation of erythroid bursts. Under optimal conditions erythroid bursts formed in the absence of added erythropoietin, but the addition of as little as 0.05 U of erythropoietin per ml to infected cultures resulted in a significant increase in numbers of erythroid bursts and the proportion of hemoglobinized cells. A comparison of the kinetics of burst formation and the size of the induced bursts with those induced with Friend virus suggested that either sarcoma virus such as the myeloproliferative sarcoma virus or the target cells for the two viruses were not the same. Density characterization and heat lability studies indicated that the increased erythroid proliferation in vitro was a virus-induced event, but the possibility that the induced erythroid burst formation is mediated via interaction with a nonerythroid target cell and subsequent release of a soluble factor cannot be ruled out.

The myeloproliferative sarcoma virus causes transformation or erythroid progenitor cells in vitro

The myeloproliferative sarcoma virus induces spleen focus formation in vivo and transforms fibroblasts in vitro. We showed in this study that in vitro infection of spleen or bone marrow cells from susceptible mice with the myeloproliferative sarcoma virus leads to the formation of erythroid bursts. Under optimal conditions erythroid bursts formed in the absence of added erythropoietin, but the addition of as little as 0.05 U of erythropoietin per ml to infected cultures resulted in a significant increase in numbers of erythroid bursts and the proportion of hemoglobinized cells. A comparison of the kinetics of burst formation and the size of the induced bursts with those induced with Friend virus suggested that either sarcoma virus such as the myeloproliferative sarcoma virus or the target cells for the two viruses were not the same. Density characterization and heat lability studies indicated that the increased erythroid proliferation in vitro was a virus-induced event, but the possibility that the induced erythroid burst formation is mediated via interaction with a nonerythroid target cell and subsequent release of a soluble factor cannot be ruled out.

Selectable retrovirus vectors encoding Friend virus gp55 or erythropoietin induce polycythemia with different phenotypic expression and disease progression

The Friend spleen focus-forming virus induces a massive expansion of erythroid progenitor cells resulting in polycythemia and splenomegaly. The pathogenic agent is the membrane glycoprotein gp55, encoded by the env gene. Recent evidence indicates that gp55 binds to and activates the erythropoietin (Epo) receptor. It is not clear, however, whether gp55 completely mimics the natural receptor ligand (Epo). To directly compare both effectors, we constructed selectable retroviral vectors which carry either the env or the Epo gene. The selection marker allowed for clonal analysis of infected cells. After infection of DBA/2J mice, the spleen weight, hematological indices, and Epo titer of peripheral blood were monitored. Although both viruses induced an acute erythrocytosis, there were significant differences in disease phenotype and progression. The Epo virus caused an enhanced increase of hematocrit and erythrocytes, whereas with the env virus the pool of late progenitors (CFU-erythroid) was dramatically expanded, resulting in a more severe splenomegaly. The distribution of cytologically recognizable erythroid precursors was shifted towards immature cell types by the env vector compared with Epo. These data suggest that Epo and gp55 differentially affect proliferation and differentiation. Gp55 appears to promote proliferation over differentiation, whereas Epo preferentially drives differentiation.

Changes in cellular gene expression in rat thyroid epithelial cells transformed by the Kirsten murine sarcoma virus

We have exploited a recently characterized system of rat thyroid epithelial cells transformed by the wild-type (wt) and a temperature-sensitive (ts) mutant strain of the Kirsten murine sarcoma virus (Ki-MSV) in order to study the effects of the K-ras oncogene on the gene expression of differentiated thyroid epithelial cells. By using cDNAs isolated from normal thyroid glands as probes, we were able to identify three sets of cellular sequences whose expression is influenced by the v-K-ras oncogene. The first set of genes is irreversibly repressed by transformation with both the wt and the ts viruses. The second set of genes is repressed in the ts-Ki-MSV-transformed cells but not in the same cells grown at the nonpermissive temperature. A third set of genes is present at higher levels at the nonpermissive temperature than at the permissive temperature. This system has allowed us to isolate and characterize a number of cDNA clones belonging to each of these three sets of genes. These specific cDNAs are suitable probes to study phenotypical changes during transformation of epithelial cells.

Expression of a fms-related oncogene in carcinogen-induced neoplastic epithelial cells

Following carcinogen exposure in vitro, normal rat tracheal epithelial cells are transformed in a multistage process in which the cultured cells become immortal and, ultimately, neoplastic. Five cell lines derived from tumors produced by neoplastically transformed rat tracheal epithelial cells were examined for the expression of 11 cellular oncogenes previously implicated in pulmonary or epithelial carcinogenesis. RNA homologous to fms was expressed at a level 5-19 times higher than normal tracheal epithelial cells in three of five of the tumor-derived lines. All three lines expressing high levels of fms-related RNA gave rise to invasive tumors of epithelial origin when injected into nude mice. Increased expression of the fms-related mRNA was not due to gene amplification, and no gene rearrangement was detected by Southern analyses. RNA blot analysis using a 3' v-fms probe detected a 9.5-kilobase message in the three tumor-derived lines, whereas both normal rat alveolar macrophages and the human choriocarcinoma line BeWo expressed a fms transcript of approximately 4 kilobases. We conclude from these data that the gene expressed as a 9.5-kilobase transcript in these neoplastic epithelial cells is a member of a fms-related gene family but may be distinct from the gene that encodes the macrophage colony-stimulating factor (CSF-1) receptor.

The expression of differentiation antigens by Rauscher virus-induced erythroid, lymphoid and myeloid cell lines

Rauscher murine leukemia virus-induced erythroid, lymphoid and myeloid cell lines were characterized with respect to the expression of differentiation antigens using a panel of monoclonal antibodies. The expression of differentiation antigens was measured in a two-step micro ELISA procedure. The cell lines express a number of early markers and lack a number of mature markers characteristic for the respective cell lineages. Moreover they express a number of surface markers which are not or only rarely found on their normal counterparts. The expression of differentiation antigens indicates that the cell lines investigated are arrested in an immature stage of differentiation. This observation implies that the Rauscher virus preferentially transforms early hemopoietic cells.

Multipotential hemopoietic cell lines isolated from stem cell cultures infected with Friend virus complex (MuLV + F-SFFV) show presence of MuLV but not F-SFFV

The only factor-dependent or factor-independent hemopoietic murine stem cell lines which can be permanently maintained in vitro are cell lines which originate from bone marrow cultures of congenic mice differing in the Fv-2 locus infected with Friend spleen focus forming virus (F-SFFV) in conjunction with either Rauscher or Friend helper virus (R-MuLV or F-MuLV). We determined the viral state of these cells by restriction enzyme analysis, measurement of SFFV and MuLV related RNA, immunoprecipitation analysis of viral related proteins and biological activity to test whether integration and expression of R-MuLV or F-SFFV are obligatory. All stem cell lines (SUT, JUT, 416B) showed expression of MuLV coded proteins or virus but SFFV was not found in these cell lines. A two-fold difference of RNA hybridizing with SFFV-specific cDNAs observed in stem cells of Fv-2rr and of Fv-2ss genotype is in agreement with data published earlier.

A mos oncogene-containing retrovirus, myeloproliferative sarcoma virus, transforms rat thyroid epithelial cells and irreversibly blocks their differentiation pattern

Differentiated, cloned rat thyroid epithelial cells (424 cells) were infected with a wild-type and a temperature-sensitive strain of the myeloproliferative variant of the Moloney murine sarcoma virus. The thyroid cells were productively infected and transformed by both virus strains and displayed some of the typical properties of malignant cells, such as morphological changes, growth in soft agar, and in vivo tumorigenicity. The acquisition of the transformed phenotype by the virus-infected cells was accompanied by a loss of the typical differentiated features of the thyroid epithelium, such as thyroglobulin (TG) secretion, iodide uptake, and dependence for growth on six factors including thyrotropin, the physiological thyroid stimulator. TG mRNA could not be demonstrated in cells transformed by both viral strains, suggesting a block at the level of the TG gene transcription. While the transformed state of the cell clones infected with the temperature-sensitive strain could be reverted by shifting the cultures to the temperature nonpermissive for transformation (39 degrees C), no reversion of the differentiated functions took place after such a shift, showing that the v-mos oncogene irreversibly shuts off the differentiation of thyroid epithelial cells in vitro. These results demonstrate, for the first time, an oncogenic potential of the v-mos oncogene family towards differentiated epithelial cells in vitro.

Lack of retrovirus gene expression in somatic cell hybrids of friend cells and teratocarcinoma cells with a teratocarcinoma phenotype

Two types of hybrids between cells with erythroid phenotype (Friend cells) and teratocarcinoma cells can be distinguished: cell hybrids with an erythroid phenotype, which release or can be induced to release large amounts of Friend spleen focus-forming virus (F-SFFV) on exposure to bromodeoxyuridine and cell hybrids with a teratocarcinoma phenotype, which do not release Friend virus and are not inducible for F-SFFV release. In this paper, we attempted to relate these differences to the expression of F-SFFV and Friend murine leukemia virus (F-MuLV) functions. Teratocarcinoma phenotype hybrids retained F-SFFV-and F-MuLV-related provirus sequences. They did not express F-SFFV- or F-MuLV-related RNA or proteins. The hybrids differentiated to endoderm-like cells on exposure to retinoic acid or hexamethylene-bis -acetamide. These cells, in contrast to the teratocarcinoma phenotype (uninduced) cells expressing SSEA-1-like antigens, did not express SSEA-1-like antigens; they formed typical, prekeratin-staining cytoskeletal structures and could be induced to release mouse interferon. The differentiating cells, but not the uninduced teratocarcinoma hybrids, were infected productively with F-MuLV or the F-MuLV--F-SFFV complex. They, however, did not express endogenous F-SFFV. Endogenous F-SFFV functions could not be rescued by infection with F-MuLV. Induction of teratocarcinoma hybrids with retinoic acid did not activate endogenous F-MuLV or F-SFFV transcription or protein synthesis. These data demonstrated two control mechanisms of Friend virus repression: one which acted trans during formation of the cell hybrids and was maintained only in teratocarcinoma phenotype cells and the other which acted cis and was still operative during induction of endodermal differentiation.

Characterization of the env gene and long terminal repeat of molecularly cloned Friend mink cell focus-inducing virus DNA

The highly oncogenic erythroleukemia-inducing Friend mink cell focus-inducing (MCF) virus was molecularly cloned in phage lambda gtWES.lambda B, and the DNA sequences of the env gene and the long terminal repeat were determined. The nucleotide sequences of Friend MCF virus and Friend spleen focus-forming virus were quite homologous, supporting the hypothesis that Friend spleen focus-forming virus might be generated via Friend MCF virus from an ecotropic Friend virus mainly by some deletions. Despite their different pathogenicity, the nucleotide sequences of the env gene of Friend MCF virus and Moloney MCF virus were quite homologous, suggesting that the putative parent sequence for the generation of both MCF viruses and the recombinational mechanism for their generation might be the same. We compare the amino acid sequences in lymphoid leukemia-inducing ecotropic Moloney virus and Moloney MCF virus, and erythroblastic leukemia-inducing ecotropic Friend virus, Friend-MCF virus, and Friend spleen focus-forming virus. The Friend MCF virus long terminal repeat was found to be 550 base pairs long. This contained two copies of the 39-base-pair tandem repeat, whereas the spleen focus-forming virus genome contained a single copy of the same sequence.

Lack of retrovirus gene expression in somatic cell hybrids of friend cells and teratocarcinoma cells with a teratocarcinoma phenotype

Two types of hybrids between cells with erythroid phenotype (Friend cells) and teratocarcinoma cells can be distinguished: cell hybrids with an erythroid phenotype, which release or can be induced to release large amounts of Friend spleen focus-forming virus (F-SFFV) on exposure to bromodeoxyuridine and cell hybrids with a teratocarcinoma phenotype, which do not release Friend virus and are not inducible for F-SFFV release. In this paper, we attempted to relate these differences to the expression of F-SFFV and Friend murine leukemia virus (F-MuLV) functions. Teratocarcinoma phenotype hybrids retained F-SFFV-and F-MuLV-related provirus sequences. They did not express F-SFFV- or F-MuLV-related RNA or proteins. The hybrids differentiated to endoderm-like cells on exposure to retinoic acid or hexamethylene-bis -acetamide. These cells, in contrast to the teratocarcinoma phenotype (uninduced) cells expressing SSEA-1-like antigens, did not express SSEA-1-like antigens; they formed typical, prekeratin-staining cytoskeletal structures and could be induced to release mouse interferon. The differentiating cells, but not the uninduced teratocarcinoma hybrids, were infected productively with F-MuLV or the F-MuLV--F-SFFV complex. They, however, did not express endogenous F-SFFV. Endogenous F-SFFV functions could not be rescued by infection with F-MuLV. Induction of teratocarcinoma hybrids with retinoic acid did not activate endogenous F-MuLV or F-SFFV transcription or protein synthesis. These data demonstrated two control mechanisms of Friend virus repression: one which acted trans during formation of the cell hybrids and was maintained only in teratocarcinoma phenotype cells and the other which acted cis and was still operative during induction of endodermal differentiation.

Continuous suppression of globin gene expression and differentiation of Friend erythroleukemia cells by phorbol 12-myristate 13-acetate (PMA) despite the loss of PMA binding sites by down regulation

The tumor promoter phorbol 12-myristate 13-acetate (PMA) reversibly inhibits hexamethylene bisacetamide-induced terminal differentiation of Friend erythroleukemia cells (FELC). We were successful in continuously inhibiting FELC differentiation by PMA up to 125 weeks (about 240 serial passages of cells in the presence of PMA). During that period, FELC can be induced to differentiate and enter terminal cell division upon removal of PMA. PMA-mediated suppression of FELC differentiation was associated with only a low level of globin mRNA accumulation. However, a rapid accumulation of globin mRNA in the cytoplasm followed by hemoglobin accumulation occurred upon removal of PMA. A specific, saturable, high-affinity receptor for phorbol esters is present in FELC, as was shown by binding studies with [3H]phorbol 12,13-dibutyrate. A significant (80%) loss in the number of phorbol ester receptors of FELC was observed after a continuous inhibition of differentiation by PMA for as much as 125 weeks. Despite such a down regulation of phorbol ester receptors, these cells respond to PMA with a dose-response similar to that of their parent cells, which have the normal number of phorbol ester receptors. Thus, PMA can suppress reversibly the accumulation of globin-specific mRNA and terminal differentiation of FELC during prolonged periods, despite loss of receptor sites, and our results suggest that only few phorbol ester receptors may be necessary for complete inhibition of FELC differentiation by PMA.

Dissociation between transformed and differentiated phenotype in rat thyroid epithelial cells after transformation with a temperature-sensitive mutant of the Kirsten murine sarcoma virus

Differentiated rat thyroid epithelial cells, infected in vitro with a temperature-sensitive mutant of the Kirsten murine sarcoma virus, expressed at the permissive temperature (33 degrees C) some phenotypic properties typical of transformed cells, including morphological features, colony formation in agar, and induction of tumors in newborn animals. Specific functional markers of these differentiated cells, i.e., synthesis/secretion of thyroglobulin, synthesis of thyroglobulin mRNA and iodide uptake, were blocked during growth at 33 degrees C. Normal morphology, failure to grow in agar, and the requirement of hormones for optimal growth were all restored after shifting to the temperature nonpermissive for transformation (39 degrees C), though the typical differentiated functions remained blocked. Infection with a leukemia helper virus clone (Moloney or Kirsten murine leukemia virus) did not lead to the loss of the differentiated phenotype of rat epithelial thyroid cells, thus demonstrating that the loss of the differentiated phenotype is caused by the sarcoma virus component. These results indicate that the expression of some of the phenotypic properties of transformed differentiated rat thyroid epithelial cells is under the direct control of the p21 thermosensitive activity, whereas the block in the expression of two typical differentiation markers of thyroid epithelial cells is irreversible and probably controlled by different mechanisms.

Retrovirus transduction: segregation of the viral transforming function and the herpes simplex virus tk gene in infectious Friend spleen focus-forming virus thymidine kinase vectors

A series of deletions and insertions utilizing the herpesvirus thymidine kinase gene (tk) were constructed in the murine retrovirus Friend spleen focus-forming virus (SFFV). In all cases, the coding region for the SFFV-specific glycoprotein (gp55), which is implicated in erythroleukemic transformation, was left intact. These SFFV-TK and SFFV deletion vectors were analyzed for expression of tk and gp55 after DNA-mediated gene transfer. In addition, virus rescued by cotransfection of these vectors with Moloney murine leukemia virus was analyzed for infectious TK-transducing virus, gp55 expression, and erythroleukemia-inducing ability. The experiments demonstrated that deletions or insertions within the intron for the gp55 env gene can interfere with expression of gp55 after both DNA-mediated gene transfer and virus infection. In contrast, the gene transfer efficiency of the tk gene was unaffected in the SFFV-TK vectors, and high-titer infectious TK virus could be recovered. Revertant viruses capable of inducing erythroleukemia and expressing gp55 were generated after cotransfection of the SFFV-TK vectors with murine leukemia virus. The revertant viruses lost both tk sequences and the ability to transduce TK- fibroblasts to a TK+ phenotype. These experiments demonstrate that segregation of the TK and erythroleukemia functions can occur in retrovirus vectors which initially carry both markers.

Retrovirus transduction: segregation of the viral transforming function and the herpes simplex virus tk gene in infectious Friend spleen focus-forming virus thymidine kinase vectors

A series of deletions and insertions utilizing the herpesvirus thymidine kinase gene (tk) were constructed in the murine retrovirus Friend spleen focus-forming virus (SFFV). In all cases, the coding region for the SFFV-specific glycoprotein (gp55), which is implicated in erythroleukemic transformation, was left intact. These SFFV-TK and SFFV deletion vectors were analyzed for expression of tk and gp55 after DNA-mediated gene transfer. In addition, virus rescued by cotransfection of these vectors with Moloney murine leukemia virus was analyzed for infectious TK-transducing virus, gp55 expression, and erythroleukemia-inducing ability. The experiments demonstrated that deletions or insertions within the intron for the gp55 env gene can interfere with expression of gp55 after both DNA-mediated gene transfer and virus infection. In contrast, the gene transfer efficiency of the tk gene was unaffected in the SFFV-TK vectors, and high-titer infectious TK virus could be recovered. Revertant viruses capable of inducing erythroleukemia and expressing gp55 were generated after cotransfection of the SFFV-TK vectors with murine leukemia virus. The revertant viruses lost both tk sequences and the ability to transduce TK- fibroblasts to a TK+ phenotype. These experiments demonstrate that segregation of the TK and erythroleukemia functions can occur in retrovirus vectors which initially carry both markers.

Dissociation between transformed and differentiated phenotype in rat thyroid epithelial cells after transformation with a temperature-sensitive mutant of the Kirsten murine sarcoma virus

Differentiated rat thyroid epithelial cells, infected in vitro with a temperature-sensitive mutant of the Kirsten murine sarcoma virus, expressed at the permissive temperature (33 degrees C) some phenotypic properties typical of transformed cells, including morphological features, colony formation in agar, and induction of tumors in newborn animals. Specific functional markers of these differentiated cells, i.e., synthesis/secretion of thyroglobulin, synthesis of thyroglobulin mRNA and iodide uptake, were blocked during growth at 33 degrees C. Normal morphology, failure to grow in agar, and the requirement of hormones for optimal growth were all restored after shifting to the temperature nonpermissive for transformation (39 degrees C), though the typical differentiated functions remained blocked. Infection with a leukemia helper virus clone (Moloney or Kirsten murine leukemia virus) did not lead to the loss of the differentiated phenotype of rat epithelial thyroid cells, thus demonstrating that the loss of the differentiated phenotype is caused by the sarcoma virus component. These results indicate that the expression of some of the phenotypic properties of transformed differentiated rat thyroid epithelial cells is under the direct control of the p21 thermosensitive activity, whereas the block in the expression of two typical differentiation markers of thyroid epithelial cells is irreversible and probably controlled by different mechanisms.

Isolation and characterization of a gp 70+ non producer Friend tumor cell clone

Viral expression was analyzed in ten cell clones of a Friend erythroleukemia cell line (HFL/b cell line [3]), which had lost its capacity to produce infectious particles. All the ten subclones were non producers but expressed spleen focus forming virus (SFFV) polypeptides in the form of p48-p50gag and gp50-gp52env. One subclone (subclone 9) expressed the gp70env of the Friend-MuLV helper component of the Friend virus complex. Comparative analysis of viral RNA expression in one gp70- subclone (subclone 2) and in the gp70+ subclone (subclone 9) was performed using specific ecotropic env gene probe and MCF/xenotropic env gene probe. In both subclones 2 and 9, the MCF/xenotropic env gene probe detected 32S SFFV genomic RNA, 20S SFFV env gene mRNA and a 34S RNA. The ecotropic env probe failed to characterize any 38S F-MuLV genomic RNA in both clones but detected 34S RNA and 24S env mRNA in the gp70+ subclone 9. These data show that expression of a complete F-MuLV genome is not required for synthesis of env gene products.

Interferon-induced alterations in membrane functions and the growth of Daudi lymphoma and Friend leukemia cells

The Friend murine erythroleukemia cell system and the Daudi Burkitt's lymphoma cell system were used to study the effect of growth-inhibitory concentrations of interferon on membrane functions. Experiments with Friend-cell clones sensitive and resistant to interferon indicated that a number of changes in membrane transport occur rapidly after the addition of interferon to sensitive cells. While no change was observed in the activity of the (Na+/K+) ATPase in Friend cells sensitive or resistant to interferon, a piretanide-inhibitable Na+,K+, 2Cl- co-transport system was specifically inhibited after interferon treatment of sensitive cells. In contrast, treatment of Daudi cells with purified molecularly cloned or standard preparations of human leukocyte interferon gave rise to no early changes in the transport of amino acids, 32Pi, sugars, or 86Rb+. The major change observed in Daudi cells was a marked reduction in the uptake and incorporation of thymidine, which begins to decrease after 8-10 h of exposure to interferon.

Effect of toyocamycin on oncornaviral production by acutely infected cells

The adenosine analogue toyocamycin incorporates into the RNA species of mammalia cells and abolishes at low concentrations of the processing of 45S preribosomal nucleolar RNA into the mature 28 and 18S cytoplasmic ribosomal RNAs. We have previously shown that toyocamycin depresses the production of the Friend leukemia viral complex by chronically infected cells. In this article, we report the study of the action of the drug on viral RNA in acutely infected cells. We found that, although abolishing viral production, the incorporation of toyocamycin does not inhibit the formation of mature viral messenger RNAs nor prevent the synthesis of specific viral proteins. These results are obtained at concentrations of analogue sufficient to abolish the appearance of mature cytoplasmic ribosomal RNA.

Effect of interferon on the replication of mink cell focus-inducing virus in murine cells: synthesis, processing, assembly, and release of viral proteins

Treatment of mink cell focus-inducing (MCF) virus (isolate AK-13) producing SC-1 cells with mouse fibroblast interferon (150 to 600 U/ml) led to a 100-fold decrease in the release of infectious virus, whereas there was a 2.5- to 10-fold decrease in various parameters of virus particle release. Analysis of labeled virion proteins indicated that a temporal change in virion protein composition occurred after interferon treatment. After a 24-h exposure of chronically infected cells to interferon, the virions produced contained a 85,000-dalton glycoprotein (apparently of nonviral origin) which was in excess of the virus envelope glycoprotein gp70. Particles produced from cells treated with interferon for 32 to 48 h were nearly devoid of gp70 and contained substantially lower quantities of p30. Intracellular processing of viral precursor polyproteins to the mature virion structural proteins was not altered in the presence of interferon. However, an accumulation of the viral p30 and p12E proteins was observed in interferon-treated cells, consistent with an increase in cell-associated virions. Immunoprecipitation analysis of the tissue culture fluids from [35S]methionine-labeled control and interferon-treated cells revealed marked decrease in p30 and p15E/p12E released after interferon treatment. In contrast, gp70 did not accumulate in interferon-treated cells, but was released into the culture medium in a form that was neither pelletable nor associated with p15E/p12E.

Analysis of the myeloproliferative sarcoma virus genome: limited changes in the prototype lead to altered target cell specificity

The myeloproliferative sarcoma virus (MPSV) derived from Moloney sarcoma virus (MSV-Mol) is a unique sarcoma virus which causes expansion of the hematopoietic stem cell compartment as well as the erythroid and myeloid cell lineages. MPSV also induces spleen focus formation in adult mice as do Friend and Rauscher viruses. Analysis of the MPSV genome on methyl mercury gels showed that the genome size is 7.0 kilobases, which is larger than the defective genome of any known MSV-Mol isolate. Hybridization analysis with specific cDNA probes showed that MPSV is a modified sarcoma virus with no sequences in the unique region of the defective sarcoma genome related to unique Friend virus sequences. The only viral sequences in the defective genome other than helper virus-related sequences are derived from the Moloney sarcoma virus genome with no new cellular sequences added. There was no evidence for induction of xenotropic virus sequences in MPSV-infected spleens of DBA/2J mice, indicating that spleen focus formation can be obtained by different mechanisms.