Among Moloney murine leukemia viruses (Mo-MuLVs) having stop codons other than UAG at the gag-pol junction, Mo-MuLV with UAA, but not with UGA, had a replication disadvantage. Mo-MuLV with a glutamine codon (CAG) at the junction did not replicate. A revertant of this virus consisted of the original virus and a virus with a deletion of the pol region. Protease and Pr65gag encoded by their respective genomes complemented each other.