We had previously identified a new family of human endogenous retrovirus-like elements, the HERV-L elements (human endogenous retrovirus with leucine tRNA primer), whose pol gene was most closely related to that of the foamy viruses. HERV-L pol-related sequences were also detected in other mammalian species. The recent cloning of the mouse Fv1 gene (S. Best, P. Le Tissier, G. Towers, and J. P. Stoye, Nature 382:826-829, 1996) has shed light on another HERV-L domain--identified as a gag gene based on its location within the provirus--which was found to be 60% identical, at the nucleotide level, to the Fv1 open reading frame (ORF). We have now cloned the murine homolog of HERV-L which, in contrast to HERV-L, displays fully open reading frames in the gag and pol genes. Its predicted Gag protein shares 43% identity with the Fv1 ORF product. Moreover, the characteristic major homology region of the capsid subdomain can be identified within both proteins, thus strongly emphasizing the gag-like origin of Fv1.