A variant of the functional MC1R allele that produces a facial 'mask' in other breeds of dogs (Em) is also present in Labradors, but since the colour of the mask is determined by the B locus, in Labradors the mask this gene produces is indistinguishable from the overall coat colour.E. K. Conant, R. Juras, and E. G. Cothran, (2011) "Incidence of the mask phenotype M264V mutation in Labrador Retrievers", Research in Veterinary Science 91: e98-e99
