JSRV Env constructs. The full-length JSRV Env structure (GenBank accession no. AF357971) is schematically presented, with the cytoplasmic tail (CT) sequence shown under the structure. The YXXM motif is underlined. For chimeras, the cytoplasmic tail of JSRV Env was replaced by that of HIV-1 (JSRV-HIV; dots represent the HIV-1 sequence omitted), 10A1 MLV (JSRV-10A1), or 10A1 MLV lacking the R peptide (JSRV-10A1R−), respectively. Shown at right are the transforming activities of corresponding JSRV Env constructs examined at the indicated weeks following DNA transfection. SU, surface subunit; TM, transmembrane subunit; SP, signal peptide; M, membrane-spanning domain; ND, not done. *, stop codon.