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Moloney sarcoma virus (MSV) was isolated from a sarcoma that appeared in Balb/c mice after injection of concentrated Moloney murine leukemia virus (M-MuLV) (Moloney 1966). MSV stocks obtained from this original isolate retain approximately three fourths of the parental M-MuLV genome. The remaining portion of the MSV genome is derived from the Balb/c mouse host. These unique MSV sequences are referred to as src (Scolnick et al. 1975; Frankel and Fischinger 1976, 1977). The MSV isolates are not identical. For example, certain variants have distinguishing properties, such as high-frequency loss of the transformed phenotype (Fischinger et al. 1972). Some cannot express the MSV gag-gene product and others produce an altered gag polyprotein (Parks et al. 1976; Robey et al. 1977). The reasons for the differences between various MSV variants isolated from the original Moloney sarcoma are not obvious. Our approach to this problem has been to compare the parental M-MuLV...
Woude et al. (1980) studied this question.