Mice that become gray with age were found among wild mice in South Australia in 1970. Initially, the condition was ascribed to a dominant mutation with variation depending in part on genetic background and in part on maternal influence. All offspring of presumed heterozygous females mated to C57BL males were gray by 10 months of age, whereas only 6/65 offspring of the reciprocal cross were gray by that age. Morse et al. since have described a graying with age traceable to a maternally transmitted leukemia virus that causes melanocyte dysfunction subsequent to infection at pre- or early postnatal stages. The virus is not endogenous and is transmitted principally in milk. It is likely that viral transmission is responsible for the condition ascribed to Ga.