The occurrence of homozygous-viable dilute–short ear (Myo5a–Bmp5) double mutants in mouse specific locus mutation experiments has generally been assumed to be the result of double non- disjunction such that the mutant inherits two copies of chromosome 9 carrying the recessive alleles from the test-stock. A homozygous viable Myo5a–Bmp5 double mutant was recovered recently in our laboratory. We were able to genetically analyse both the Myo5a–Bmp5 region and proximal and distal markers in the original mutant as well as in offspring of the original mutant. Our results indicate the mutational event to be due to mitotic recombination and not double non-disjunction.