We isolated the mas proto-oncogene from a mouse genomic library. Sequence analysis showed that it contains an open reading frame without intervening sequences. The amino acid sequence deduced confirms the seven-transmembrane-domain structure and exhibits 97% and 91% amino acid homology with the rat and the human Mas, respectively. In mice and rats, mas mRNA was detected in the testis, kidney, heart, and in the brain regions: hippocampus, forebrain, piriform cortex, and olfactory bulb. Testicular mas mRNA from rats increases markedly during development, while cerebellar mRNA is high postnatally but completely disappears at later stages. We conclude that the product of the mouse mas gene may be involved in the development of the brain and testis.