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A Friend mink cell focus-inducing (Fr-MCF) virus isolated from a Friend tumor cell line was able to induce acute erythroleukemia associated with polycythemia when injected as a Friend murine ecotropic leukemia virus (F-MuLV) pseudotype into adult Swiss and ICFW mice. One virus isolate recovered from leukemic cells and designated as FV-F3 presented the following properties: (i) persistence of the same leukemogenic power when propagated in vivo and in vitro; (ii) in vivo spleen focus-forming (SFFV) capacity; (iii) presence of erythropoietin (EPO)-independent CFU-E in leukemic animals; (iv) expression of a 32 RNA specifically recognized by a SFFV probe, in FV-F3 infected cells; and (v) expression in FV-F3-infected cell of polypeptides in the range of gp52 SFFV. Peptide analysis of these products revealed close similarities with the parental MCF virus. These data suggest that a SFFV genome arose by genetic recombinational events involving MCF virus.

RNA was prepared from the murine leukemia virus pseudotype of mouse sarcoma virus. On polyacrylamide gels, 70 S single-stranded RNA was detected as well as smaller RNA species of 28 S, 18 S, and 4 S. In addition, two components of 8 S were discovered. These 8 S RNA molecules may correspond to the 7 S RNA of Rous sarcoma virus and to the 9 S of avian myeloblastosis virus.