Abstract The human R- ras gene was isolated by low-stringency hybridization with a v-H- ras probe. The predicted 218 amino acid R- ras protein has an amino-terminal extension of 26 residues compared with H- ras p21, and shows 55% amino acid identity; conserved domains include the p21 GTP-binding site and the carboxy-terminal membrane localization sequence. R- ras has at least six exons, with the position of the first intron conserved relative to the Drosophila ras64B and Dictyostelium ras genes; there is no similarity in the exon-intron structure of the R- ras gene and of the mammalian H-, K-, and N- ras proto-oncogenes. Cloned mouse R- ras cDNAs exhibit 88% nucleotide and 94.5% predicted amino acid identity to human R- ras. Human R- ras was localized to chromosome 19, a site different from ras p21 genes. Mouse R- ras is syntenic with c-H- ras on chromosome 7.