The SELH/Bc inbred mouse stock has produced an unusually high number of spontaneous mutations, including sph2Bc, nuBc, a recessive lens opacity, and three mutations at the c locus. Classical genetic and molecular genetic studies were done to investigate the origin of the albino locus mutations. Southern blots probed with the mouse tyrosinase cDNA showed that two of the mutations, cBc and c2Bc, are deletions of exons 1, 2 and 3. The third mutation, c3Bc, showed a disruption, either a rearrangement or an insertion, in the region of exon 1. The deletion of the cBc mutation is anticipated to be large as the mutation has inactivated the Mod-2 locus 2 cM away, and an essential locus for postimplantation survival outside the c–Mod-2 interval, whereas the c2Bc mutation is viable and fertile in homozygotes. Homozygotes for c3Bc are also viable and fertile. We conclude that at least some of the molecular events leading to the three albino mutations were independent. The mutations differ from each other and from the classical albino point mutation. All three new mutations originated from parents who were germline mosaics, and the mutational events were therefore all postmeiotic. All three mosaics shared one common ancestor six generations previously, raising the possibility that an instability of the albino locus might have been inherited. SELH/Bc mice may provide an animal model for the study of mechanisms underlying genetic instability.Key words: mutation, tyrosinase, mosaicism, deletion.