The v-mosm1 nucleotide sequence codes for a protein that is 376 amino acids long. Although the N-terminus is homologous with that of the v-mos124 protein, the C-terminus is substantially different from the C-termini of all other examined mos proteins, suggesting that this region is nonessential and perhaps cleaved. Overall, v-mosm1 has greater homology with c-mos than does v-mos124, but mutually exclusive differences between c-mos and each of the v-mos genes preclude linear descent and suggest a common ancestral murine sarcoma virus.