The complete nucleotide sequence of the gene for the specific glycoprotein (gp55) of the polycythemic strain of Friend spleen focus-forming virus (SFFV) The spleen focus-forming virus (SFFV), which is contained in the Friend leukemia virus preparation (1), is a replication-defective murine retrovirus. When inoculated as a pseudotype, SFFV causes rapid splenomegaly and erythroleukemia in susceptible adult mice (2). Depending on the SFFV strain the erythroleukemia is accompanied by either anemia or polycythemia. SFFV is considered a member of a diverse group of the acute leukemia and sarcoma viruses of mammalian and avian origin, and it is distinguished in that it has a strict target cell specificity both in vivo and in vitro and has no unique sequence of cellular origin (3).For the elucidation of the mechanism of erythroleukemia induction by SFFV, the identification of the leukemogenic sequence and its protein product is the first step. Two specific proteins have been shown to be coded for by SFFV. One is a gag gene-related protein, and the other is an env gene-related glycoprotein (gp55; also designated gp52) (4-7). The gag generelated protein (p45) is coded for only by a certain strain of SFFV (8). In contrast, gp55 is observed in the cells infected with any SFFV strain and has been relatively well characterized in the examination of its role in leukemogenesis (3, 9). Although the molecular size and details of the peptide map of gp55 are somewhat variable depending on the SFFV strain (10), the gp55s coded for by different SFFV strains have a common structural property: they are immunologically crossreactive with gp7O, the major viral envelope glycoprotein, of the ecotropic murine leukemia virus (MuLV) (4-6). Analysis of the SFFV genome 32S RNA by molecular hybridization (11), oligonucleotide fingerprinting (12, 13), or heteroduplex formation (14)