The structure of Id was examined by Southern analysis in inbred mouse lines which included five subspecies of Mus musculus. No variation was detected within this species. The species Mus cookii shares the same form found in mice of M. musculus derivation, indicative of a long evolutionary history and a common origin. However, five TaqI polymorphisms were found among several Mus species, suggesting that Id has been modified throughout species diversification. Whether these variants impart functional changes is yet to be determined.