There is unequivocal evidence that a relatively nonpolymorphic class I gene (designated Q10) from the Qa region of inbred mice encodes a secreted class I molecule. We have used a cDNA probe specific for this gene and an antiserum specific for its secreted protein product to investigate the occurrence and expression of this gene in different species of wild mice broadly representing the entire genus Mus. Evidence is presented that a Q10-like gene has been conserved and is transcribed and translated throughout the genus, suggesting that it serves an important function. However, the data also show that some differences have appeared in this gene over the period of evolutionary time covered by this sampling of wild mice. These results indicate that a specific class I DNA probe isolated from inbred mice can be used to discriminate a particular gene among the multiple class I genes present in other species.