We previously showed that in vitro translation of M-MuSV virion RNA yielded a 62 kd gag gene product and an overlapping set of four proteins with approximate molecular weights of 37,000, 33,000, 24,000 and 18,000. In this paper we show, by use of hybrid arrest translation with cloned recombinant DNAs containing M-MuSV v-mosMo sequences, that the 37, 33, 24 and 18 kd proteins are synthesized in their entirety from the v-mosMo gene. Analysis of the primary sequence of these proteins shows that each one is initiated independently from AUG codons within the v-mosMo gene and utilizes the long open reading frame predicted from the v-mosMo DNA sequence. Antisera against synthetic peptides corresponding to the C terminus of the predicted v-mosMo gene product precipitate all four in vitro v-mosMo proteins.