The serological properties of the gag gene products p15 and p12 of N-and B-tropic viruses of C57BL mice have been examined. Although these viruses were serologically identical by competition assays for proteins gp7l and p30, they were readily distinguishable in competition assays for proteins p15 and p12. Two isolates of N-tropic viruses had pi2s serologically indistinguishable from AKR murine leukemia virus p12, while two Btropic isolates had distinctly different p12s. The latter pi2s were serologically indistinguishable from the p12 purified from the Btropic radiation leukemia virus (RadLV)-/VL-3. Moreover, this p12 was indistinguishable from the p12 of the endogenous C57BL/Ka xenotropic virus. Similarly, the piSs of the B-tropic viruses were serologically distinct from the AKR murine leukemia virus type of p15, as was the p15 of one C57BL N-tropic virus, while another N-tropic isolate had a p15 identical to the AKR murine leukemia virus p15. These results are interpreted to suggest that the endogenous N-tropic virus of C57BL mice undergoes recombination with the endogenous, xenotropic virus and that this mechanism is involved in the generation of B-tropic viruses in C57BL mice.

C3H/Sm mice have lost the exogenous milk-borne mouse mammary tumor virus C3H/Sm mice (previously designated C3H/ viral sequences into RNA (24). The incidence of StWi) spontaneously lost their exogenous milk-spontaneous mammary cancer in uninfected borne mouse mammary tumor virus (MMTV), C3H/Sm mice is only 1.5%, the tumors develop-MMTV (C3H) or MMTV (S), in 1958 and be-ing in very old, retired breeders at an average came a low-mammary-cancer subline (22). Un-age of 23 months. C3H/Sm mice are susceptible like other MMTV (C3H)-free C3H sublines, to mammary tumorigenesis by the chemical carhowever, they do not ordinarily express MMTV cinogens 7,12-dimethylbenz[a]anthracene antigens or complete virions although they ac-(DMBA) and urethane, but they are resistant to tively transcribe their endogenous MMTV pro-the development of an increased rate of mamma-876 on July 6, 2020 by guest

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