Different serotypes of B-tropic murine leukemia viruses and association with endogenous ecotropic viral loci

We have previously presented evidence suggesting that the B-tropic viruses isolated from C57BL mice are generated through recombination of the endogenous N-tropic virus with an endogenous xenotropic virus. To further study the origin of B-tropic viruses and to genetically map the determinants of N/B-tropism, we have isolated 22 new N- and 13-tropic viruses from several strains of mice and have characterized their gene products serologically. For comparison, we have characterized an additional 14 N- and B-tropic viruses obtained from other sources. All of the N-tropic viruses had AKR-like p12s and p30s which competed in a radioimmunoassay specific for endogenous N-tropic p30s. Eight of nine 13-tropic viruses from three different substrains of C57BL mice had xenotropic-type p12s demonstrating that recombination involved the gag gene region. In contrast, 8 of 8 B-tropic viruses from BALB/c mice were characterized by AKR-like p12s. All 23 of the B-tropic viruses in the study have p30s that fail to compete in the specific p30 assay. These results demonstrate that there is strain specificity in the type of recombination resulting in generation of B-tropic viruses, although in both cases the p30 region is involved. The influence of the endogenous N-tropic viral locus on the type of B-tropic viruses is indicated by the observation that 3 of 4 13-tropic viruses isolated from C57BL/6 mice carrying the C3H ecotropic viral locus resembled the BALB/c 13-tropics, rather than the C57BL B-tropic viruses.     

Polymorphism in the major core protein (p30) of murine leukemia viruses as identified by mouse antisera.

Antisera prepared in C57BL/6 mice against the AKR K36 leukemia cells detect] group-specific and class-specific antigenic determinants on the major core protein (p30) of murine leukemia virus (MuLV). Sera that contain a predominance of antibodies against class-specific antigens react preferentially with the p30 proteins of ecotropic MuLV; these sera do not react with the p30 proteins of xenotropic MuLV. The use of MuLV class-specific antisera in immunological assays enables the assignment of p30 antigens in viruses and tissues to ecotropic or xenotropic origins.     

Polymorphism among the major core proteins of C57BL B-tropic murine leukemia viruses

Previous studies have demonstrated a correlation between changes in the major core protein (p30) of murine leukemia viruses (MuLV) and their Fv-1-mediated host range. In this report, we have examined the p30 proteins of B-tropic viruses representing two serologically distinguishable classes. These proteins differed from each other as well as from prototype N-tropic and xenotropic viruses. This polymorphism, considered with serological typing of the adjacent p12 polypeptide, suggests that a crossover within the region coding for p30 is involved in the generation of these B-tropic viruses.     

Naturally occurring leukemia viruses in H-2 congenic C57BL mice. III. Characterization of C-type viruses isolated from lymphomas induced by milk transmission of B-ecotropic virus

The prevalence of different host-range classes of murine leukemia virus (MuLV) was studied in C57BL mice with (V+) and without (V-) milk transmission of a naturally occurring B-tropic ecotropic MuLV. Virus isolates were studied with respect to growth properties, XC-plaque formation, antigen profiles of their envelope proteins (gp70 and P15(E)), gp70 tryptic-peptide maps, and their potential to induce lymphomas after inoculation into newborn mice. B-tropic ecotropic MuLV with the capacity to cause plaques in XC cells was isolated from almost all lymphomas of both V+ and V- sublines. The reaction patterns of these ecotropic isolates with monoclonal antibodies reactive with MuLV-env proteins and the tryptic-peptide maps of the gp70 molecule indicate that they are similar to each other and differ only slightly from the ecotropic MuLV in the spleens of young V+ animals, which is identical to the milk-transmitted virus. XC-, B-tropic dualtropic mink cell focus-inducing (MCF) viruses were isolated from the majority of different types of lymphoma (B cell, T cell, or neither B nor T cell derived), but not from the spleens or milk of young V+ or V- animals. The env proteins of the MCF isolates are highly heterogeneous, but most isolates originating from B10.AV + T-cell lymphomas share MCF-related epitopes in their gp85 envelope precursor with AKR MCF-247 virus. Most MCF viruses isolated from non-T lymphomas do not possess these epitopes. The results indicate that also in this model the generation of dualtropic MCF viruses might be important in lymphoma induction, although only some of the cloned MCF viruses show enhanced oncogenic properties in comparison with ecotropic isolates. A cloned oncogenic MCF virus induced different lymphoma types in C57BL/10 (= B10, H-2b) and B10.A (H-2a) mice, similar to what was found earlier with the milk-transmitted virus. Hence, the lymphoma-type differences are not due to differences in the B-tropic ecotropic viruses transmitted through the milk in these strains, but reflect an influence of the H-2 complex on the phenotype of the virus-induced lymphomas.     

Clonal cells lines from a feral mouse embryo which lack host-range restrictions for murine leukemia viruses.

Tissue culture cell lines highly sensitive to both N- and B-tropic host-range variants of mouse-tropic murine leukemia virus (MuLV) were derived by clonal selection from a feral mouse embryo cell culture line. The clonal lines are more sensitive than standard assay cultures, particularly for isolating naturally occurring N- and B-tropic MuLV from tissue extracts. Also, they can support the replication of xenotropic MuLV to a limited extent.The host-range patterns of N- and B-tropic viruses are not altered by serial passage in the sensitive cells.It is presumed that these cell lines lack the Fv-1 gene function, and it is suggested that Fv-1 may have a much broader inhibitory effect than previously recognized.     

Role of endogenous murine leukemia virus in immunologically triggered lymphoreticular tumors II. Isolation of B-tropic mink cell focus-inducing (MCF) murine leukemia virus

Previous work has shown that (BALB/c x A)F, (CAF₁) mice with an immunological disorder, the graft-versus-host reaction (GVHR), express an oncogenic murine leukemia virus (MuLV) which appears to be responsible for the subsequent development of lymphoreticular tumors. A mink cell focus-inducing (MCF) virus has been isolated from a reticulum cell neoplasm induced in a BALB/c mouse by serially passaged B-tropic MuLV originating in a CAF, GVHR mouse. The cloned MCF virus has a dual host range. It grows both in mouse cells where it is XC negative, B tropic, and in mink lung cells where it induces characteristic foci. It is partially neutralized by xenotropic MuLV antiserum and it partially competes in a homologous ecotropic gp70 radioimmunocompetition assay. Both ecotropic and xenotropic MuLV interfere with its infectivity. In these biological properties, the GVHR MCF virus closely resembles recombinant AKR MCF virus. We discuss the possible origin and significance of a presumptive recombinant MuLV in a low virus model where lymphoreticular tumors are triggered by an immunological disturbance.     

A new class of retrovirus present in many murine leukemia systems

A new class of MuLV has been detected and isolated from normal and leukemic AKR, C58, SJL, and NFS.AKV mice as well as from NFS mice inoculated with Friend or Moloney ecotropic viruses. These new viruses are XC negative and serologically cross-react with MCF env antigens but are ecotropic in host range, being able to only infect mouse cells to varying degrees and unable to infect mink or other cells infectable by MCF or xenotropic viruses. Viruses of this type from AKR mice cross-interfere with Moloney ecotropic and MCF viruses in SC-1 cells and appear to have properties similar to those of the SL3-2, GPA-V2, and R-XC- isolates. Analysis of their genomes by restriction endonuclease mapping of proviral DNA indicates structures similar to class II MCFs with the 5' half of the genome being like ecotropic viruses and the env region exhibiting restriction sites characteristic of MCF viruses. In normal AKR mice, these ecotropic recombinant-like viruses are found in spleen and bone marrow as early as 1 week of age, but first appear in the thymus at 3-4 months of age. These viruses have not been detected in mice with no or low expression of ecotropic viruses (NFS, NZB, DBA/2, BALB/c, C57BL/6). Because of their apparent recombinant structure and ecotropic host range we have provisionally designated them ecotropic recombinant virus (ERV) to distinguish them from the MCF class of recombinant MuLV.     

Heterogeneity of p15(E)-related polypeptides expressed by MuLV-infected cells

The p15(E)-related polypeptides of radiation leukemia virus (RadLV)-derived viruses and of cells infected with prototype MuLV were analyzed by immunoprecipitation, SDS-PAGE, and immunofluorescence analyses. It was found that the p15(E)-related molecules of ecotropic and xenotropic viruses derived from RadLV lymphoma cell lines were distinguishable by reactivity with monoclonal anti-p15(E) antibodies and by SDS-PAGE profile. Ecotropic MuLV of RadLV origin encoded the p15(E)a antigen and produced a Pr15(E) of 20K MW. In contrast, xenotropic virus derived from RadLV did not express the p15(E)a antigen and by SDS-PAGE its Pr15(E) migrated at 21K. A previously undescribed, p15(E)-related molecule of 18.5K MW was associated with xenotropic RadLV. These differences were also reproduced by the prototype ecotropic, xenotropic, and dualtropic viruses.     

Host range conversion of murine leukemia virus resulting from recombination with endogenous virus

Summary. Ecotropic murine leukemia viruses (MuLVs) are classified into B-, N-, or NB-tropic MuLV by their host range determined by the Fv-1 gene product. B-tropic MuLV is restricted in N-type mouse cells (Fv-1^n/n) and N-tropic MuLV is restricted in B-type mouse cells (FV-1^b/b). Although forced passages in a restrictive host grant a wider host range (NB-tropism), we show here a host range conversion from B to N tropism. The conversion was most likely a result of recombination between the exogenously infected B-tropic MuLV and an endogenously expressed N-tropic MuLV in a C57BL/6 mouse cell line, YH-7. Received June 19, 1996 Accepted August 24, 1996     

Effect of murine host genotype on MCF virus expression, latency, and leukemia cell type of leukemias induced by Friend murine leukemia helper virus

Leukemias induced by neonatal inoculations of several mouse strains with different strains of Friend murine leukemia helper virus (F-MuLV) were followed for time of disease onset, cytochemical analysis of predominant cell types in leukemic organs, and expression of infectious mink cell focus-inducing (MCF) viruses detected by mink cell foci or MCF-specific monoclonal antibodies. Most BALB.B and IRW mice had a rapidly appearing, severe anemia and hepatosplenomegaly consisting of erythroid cells. MCF viruses were usually isolated from enlarged spleens of IRW mice. In contrast, C57BL/10 mice had a lower incidence of disease and much slower course. Splenomegaly and lymphadenopathy with mild anemia were seen, and the predominant cell types were either myeloid (chloroleukemia) or lymphoid. MCF viruses were never isolated from this mouse strain. (C57BL/10 X IRW)F1 mice were intermediate in latency, but all mice had disease by 8 months. Myeloid, lymphoid, and some mixed leukemias with an erythroid component were observed, but in no case did we see the severe anemia or pure erythroid involvement typical of IRW and BALB.B mice. MCF viruses were, however, isolated from 22% of these mice regardless of leukemia cell type. DBA/2 mice had a disease pattern similar to the (C57BL/10 X IRW)F1 mice, and MCF viruses were isolated from three of six mice tested. Inoculation of IRW mice with the low virulence B3 strain of F-MuLV produced disease with a longer latency than F-MuLV 57, but similar cell types were transformed by both viruses. In vitro cell lines were derived from 14 mice, and most were tumorigenic in vivo. Three lines released infectious MCF virus, and three others expressed MCF-specific cell surface antigens but did not release virus. Eight lines expressed no MCF infectious virus or viral antigens. Several lines released infectious xenotropic viruses and/or expressed xenotropic MuLV cell surface antigens recognized by monoclonal antibodies reactive with xenotropic viruses. The lack of MCF expression in many primary leukemic tissues as well as in in vitro derived leukemia cell lines of C57BL/10 and (B10 X IRW)F1 mice suggested that MCF virus generation and expression may not be required for leukemogenesis in some mouse strains or in some hemopoietic lineages.     

B-MuX: A unique murine C-type virus containing the “env” gene of xenotropic viruses and the “gag” gene of the ecotropic virus

A unique C-type virus, B-MuX, was isolated from nonproducer, BALB/c-transformed fibroblasts after IdUrd induction. B-MuX has the envelope gp70 properties of the endogenous BALB:virus-2 genome as determined by neutralization, interference, host-range characteristics and competition radioimmunoassays. Tryptic peptide analysis of gp70 from B-MuX revealed that B-MuX gp70 contains peptides in common to xenotropic virus gp70s and overall was very similar in structure to the xenotropic BALB: virus-2 gp70. In contrast, the gag gene products are serologically identical with the p15 and p12 of the endogenous, ecotropic virus. The p30 of B-MuX is structurally similar to that of the endogenous, ecotropic virus, and differs from the p30 of BALB/c xenotropic viruses as determined by tryptic peptide analysis. The results suggest that either B-MuX is a new type of endogenous, xenotropic virus or a recombinant between the endogenous xenotropic and ecotropic viruses. The unaltered xenotropic behavior of B-MuX demonstrates that the gag gene region does not contribute to this virological property.     

Structural heterogeneity in p30 molecules of type C viruses

Tryptic digests of p30 proteins from mouse type C viruses were subjected to cation-exchange chromatography. Structural heterogeneity of p30 molecules was seen in two specific areas of the peptide elution profiles. These hypervariable regions of p30 proteins were used to discriminate representative ecotropic (N- and B-tropic), xenotropic (alpha and beta) and amphotropic viruses.     

Different recombinant murine leukemia viruses use different cell surface receptors

Retroviruses can be grouped by viral interference measurements into classes which use common cell surface receptors. We previously tested a large number of isolates of mink cell focus-inducing (MCF) murine leukemia viruses (MuLVs), and reported that all of them share a distinct receptor on NIH/3T3 cells (A. Rein, Virology 120, 251, 1982). We now extend this generalization to several additional recombinant isolates, including two (SL3-2 and GPA-V2) which would not be considered MCFs on the basis of host-range data. We note the superiority of interference tests, based on positive, unambiguous data, over host-range tests for virus classification. We also show that in contrast to the MCFs, which are all derived from ecotropic MuLVs, a recombinant derived from wild mouse amphotropic MuLV (S. Rasheed et al., Int. J. Cancer 29, 345, 1982) uses a unique receptor on NIH/3T3 cells. This suggests that (a) mouse cells contain more than one type of endogenous env sequence; and (b) there is some specificity in the generation of recombinants, since ecotropic MuLVs appear to give rise only to MCFs, while amphotropic MuLV has generated a distinct type of recombinant. It also represents a second case (in addition to the MCFs) in which an env gene recombinant is more pathogenic than its exogenous parent. We also show that xenotropic MuLV does not interfere with MCFs in NZB mouse cells; thus, despite the close homology between MCF and xenotropic env sequences, the gp70 of xenotropic MuLV appears to have no detectable affinity for the MCF receptor.     

Different murine cell lines manifest unique patterns of interference to superinfection by murine leukemia viruses

Interference to superinfection by murine leukemia viruses (MuLV) was analyzed in cells chronically infected with other MuLVs. A new sensitive focal immunofluorescence assay employing monoclonal antibodies was used to detect foci of virus infection in live cell monolayers. Monoclonal antibodies were chosen which reacted with the challenge virus but not with the interfering virus. The results obtained confirmed some of the findings of previous workers using Moloney sarcoma virus pseudotypes as challenge viruses on mouse and nonmouse cells. In addition, SC-1 mouse cells nonproductively infected with defective spleen focus-forming virus were found to be resistant to superinfection by recombinant dual-tropic viruses. Furthermore, results indicated that interference patterns between some pairs of viruses differed in different cell types. Thus, xenotropic MuLV blocked superinfection by recombinant dual-tropic viruses in SC-1 feral mouse cells, but not in two lines of NZB mouse cells. Also, in a Mus dunii tail fibroblast cell line some unique patterns of interference were observed. One ecotropic MuLV blocked infection by two xenotropic viruses and three recombinant dual-tropic viruses. Two other ecotropic viruses blocked infection by only one of the two xenotropic viruses tested. These two ecotropic viruses also differed from each other in their ability to block the three recombinant viruses. In addition, two strains of amphotropic MuLV also differed in their interference capacity. As expected, strain 1504A did not block any viruses tested, whereas strain 4070A surprisingly blocked one xenotropic and one ecotropic MuLV. The lack of homogeneity in interference patterns seen in the Mus dunii cells suggested either that a large number of heterogeneous virus receptors were present on this cell line or that interference in these cells might operate through a mechanism other than blocking of virus receptors by the envelope protein of the interfering virus.     

Phenotypic mixing between murine leukemia viruses: characteristics of ecotropic virus infection of heterologous cells.

Ecotropic murine leukemia viruses (MuLV) were capable of infecting mink, rabbit, duck, guinea pig, human, and cat cells, but not chicken cells, when phenotypically mixed with xenotropic MuLV. Mink cell cultures inoculated with the phenotypically mixed pools showed XC plaques with one-hit dose response kinetics; these plaques represented colonies derived from outgrowth of single infected cells rather than spreading areas of infection, which showed two-hit kinetics. Clonal lines of mink and rabbit cells chronically infected with N-tropic ecotropic (AKR-L1), B-tropic ecotropic (WN1802B), and NB-tropic ecotrpic (Friend) MuLV were established. Production of virus by these xenogenic cell lines was generally much lower than by mouse cells, as was the number of cells with gs-antigen detected by immunofluorescence. However, essentially all cells registered as virus-producers on infectious center plating. Treatment with IUdR enhanced virus production by 10-fold, and immunofluorescence staining as well, in the three infected mink cell lines. The viruses retained their ecotropic and Fv-1-determined host range characteristics through more than 6 to 12 months of chronic virus production in the heterologous cells.     

Biological, immunological, and biochemical evidence that HIX virus is a recombinant between moloney leukemia virus and a murine xenotropic C type virus

Various parameters of HIX virus were examined to determine its origin and its relationship to other murine C type oncornaviruses. Envelope properties of HIX virus grown in cells of several species were subjected to analyses of host range, interference, and neutralization. Cloned amphotropic HIX virus was adapted to grow in human RD cells. After 6 months in culture, the resulting virus (HIX-RD) could enter mouse cells but essentially lost the capacity of propagating in mouse cells. Interference patterns of HIX and HIX-RD were identical to each other and unrelated to murine ecotropic MuLV interference. MSV(HIX) or MSV(HIX-RD) could not penetrate HIX-, HIX-RD-, or MuX-preinfected cells. However, infection with HIX exhibited a unique one-way interference in that MSV(MuX) could penetrate and transform HIX-preinfected cells. Neutralization of HIX and HIX-RD with relatively type-specific anti-gp70 sera showed that they resembled Moloney (M)-MuLV most closely. Significant neutralization was observed also with anti-Rauscher gp70 or BALB-2 MuX gp70 sera. Both HIX derivatives were acutely susceptible to inactivation with normal mouse sera, a characteristic of xenotropic viruses. Competition radioimmunoassays were performed to determine the antigenic relationship of HIX to other MuLV types. The highly type-specific phosphorylated p12 and the relatively type-specific gag region p15 of HIX were found to be identical to M-MuLV and less related to other murine C-type oncornaviruses. The examination of HIX gp70 with type-specific anti-M-MuLV or anti-C57L MuX gp70 sera showed that it was clearly different from either virus. Tryptic peptide maps of the gag region-p15 and p30 of HIX were identical to corresponding maps of M-MuLV proteins. The gp70 of HIX was unique and different from known eco-, xeno-, and amphotropic murine C type oncornaviruses. Based on known migration patterns of characteristic trypsin- and chymotrypsin-derived peptides of various eco-, and xenotropic MuLV's, it was concluded that gp70 of HIX was related to both MuX and M-MuLV. Tryptic fingerprint maps also revealed several significant differences between parental HIX and its HIX-RD variant. Comparative hybridizations of assorted high-molecular-weight (HMW) virus RNAs with complementary DNA from HIX virus showed that, with unfractionated probes, no significant differences could be seen between HIX and M-MuLV. Based on the above, HIX virus appears to contain predominantly M-MuLV-specific information except for its envelope gene which has been presumably derived from a recombinational event involving corresponding M-MuLV and MuX nucleotide sequences.     

Physicochemical, biological and serological properties of a leukemogenic virus isolated from cultured RadLV-induced lymphomas of C57BL/Ka mice.

RadLV/VL₃, a virus spontaneously and continuously produced at high titer by BL/VL₃ cells, a line established in culture from a RadLV-induced C57BL/Ka mouse thymic lymphoma, has been extensively characterized. Evidence is presented indicating that it is a C-type RNA virus with highly thymotropic and leukemogenic biologic properties identical with those of its RadLV parent. It was not significantly neutralized by antibodies prepared against other ecotropic or xenotropic murine C-type viruses, used individually or in combination, whereas these antibodies effectively neutralized MCF-247 virus. However, antisera prepared against RadLV/VL₃ in rabbits and mice readily neutralized RadLV/VL₃. Curiously, the rabbit antiserum also had strong neutralizing activity against three other C-type endogenous viruses of the C57BL/Ka strain, BL/Ka(B), BL/Ka(N), and BL/Ka(X), all of which are fibrotropic (replicate readily in fibroblasts of appropriate tropism) and lack thymotropic and leukemogenic activity. In interference assays, these viruses and RadLV/VL₃ exhibited little or no capacity for mutual or self-interference, nor did RadLV/VL₃ interfere with the replication of MCF-247 virus in mink cells. However, the addition to cultures of increasing concentrations of the purified envelope glycoprotein (gp71) of RadLV/VL₃, though devoid of effect on BL/Ka(B) and BL/Ka(N), strongly inhibited the replication of RadLV and RadLV/VL₃ both in vitro and in vivo, and partially inhibited the xenotropic replication of BL/Ka(X). Competition radioimmunoassays with purified virion polypeptides demonstrated that the gp71 of RadLV/VL₃ is distinctively different from that of all other viruses tested; that the p12's of RadLV/VL₃ and BL/Ka(B) are similar or identical, whereas that of BL/Ka(N) competes in the AKR-MuLV p12 assay; and that the p30's of all of the viruses studied are indistinguishable by this method. Finally, RadLV/VL₃ failed to elicit cytopathic changes or transformed cell foci after serial passage in mink lung cells, even after co-infection with the xenotropic BL/Ka(X) virus, indicating that it is apparently devoid of MCF activity. Thus, MCF activity is apparently not a prerequisite for thymotropic and leukemogenic activity, at least in the C57BL/Ka strain.     

Demonstration of two immunologically distinct xenotropic type C RNA viruses of mouse cells

Several independent type C virus isolates were obtained by transplantation of human tumor cell lines into immunosuppressed NIH Swiss mice. Each isolate appeared to be of mouse origin as determined by the antigenicity of its major virion polypeptide, p30. The virus isolates were indistinguishable from each other in serologic and host range properties and also closely resembled BALB:virus-2, one endogenous virus of $\text{BALB}\text{c}$ mouse cells. However, in immunoassays for a highly type-specific virion polypeptide, designated p12, every isolate was found to differ from BALB: virus-2. These results provide evidence for the existence of immunologically distinct xenotropic viruses of mouse cells.     

应用基因重组病毒表达产物筛选FBL3小鼠白血病肿瘤特异性抗原表位

肿瘤特异性抗原是肿瘤免疫研究的中心课题，众多的研究表明Ｔ细胞在肿瘤免疫中起重要作用，因而寻找Ｔ细胞所识别的肿瘤特异性表位并以此为瘤苗的靶抗原有重要的研究价值。以一系列重组Ｆ－ＭｕＬＶｇａｇ病毒载体，用已建立的ＦＢＬ３－ＣＴＬ克隆筛选，并通过Ｈ－２Ｉ类分子结合实验，精确地测得Ｆ－ＭｕＬＶｇａｇ中一个能被为ＦＢＬ３－ＣＴＬ克隆所识别的表位（ＣＣＬＣＬＴＶＦＬ，ｐ８５～９３）。并以此ｐ８５～９３合成肽免疫动物成功诱导出能识别亲本ＦＢＬ３肿瘤细胞。     

Origin of mink cytopathic focus-forming (MCF) viruses:comparison with ecotropic and xenotropic murine leukemia virus genomes

Restriction endonuclease maps have been developed for the viral DNAs from nine xenotropic and eleven MCF murine leukemia viruses (MuLV) isolated from AKR and other mouse strains. In contrast to the highly related nature of ecotropic viral DNAs isolated from inbred mice and from M. m. molossinus, each xenotropic and MCF viral DNA was unique. Xenotropic MuLV DNAs could be divided into two classes, which correlated with previously reported serological and biochemical data; one xenotropic MuLV isolated from an AKR mouse showed features of both classes. Ecotropic viral DNA hybridized poorly or not at all to a 1.2 kbp segment of xenotropic viral DNA located in env 6.3–7.5 kbp from the left end of the viral DNAs. All MCF viral DNAs contained noneco-tropic sequences in a portion of the env gene region, but some MCF viruses were composed principally of nonecotropic sequences. The nonecotropic regions of the MCF viral DNAs were related to xenotropic MuLV DNA, but many MCF viral DNAs contained sequences not found either in xenotropic or ecotropic MuLV DNA. It was concluded that these MCF viruses probably arose via recombination between ecotropic MuLV and endogenous MuLV DNA sequences; sequences of recombination include a portion of env, but need not be limited to this region. The polytropic host range of MCF viruses may represent an endogenous viral function.