Definition: Strains of MURINE LEUKEMIA VIRUS that are replication-defective and rapidly transforming. The envelope gene plays an essential role in initiating erythroleukemia (LEUKEMIA, ERYTHROBLASTIC, ACUTE), manifested by splenic foci, SPLENOMEGALY, and POLYCYTHEMIA. Spleen focus-forming viruses are generated by recombination with endogenous retroviral sequences.
infection: coordinate IM with RETROVIRIDAE INFECTIONS (IM) + TUMOR VIRUS INFECTIONS (IM) + LEUKEMIA, EXPERIMENTAL (IM) or LEUKEMIA, ERYTHROBLASTIC, ACUTE (IM) (for spontaneous erythroleukemia)