The Em variant of the MC1R gene at the extension (E/e) Locus results in the localized distribution of eumelanin (black pigment) on the muzzle of the dog which resembles a dark "mask" on dogs with lighter colored coats.

Although the Em variant produces a black melanistic mask, the final color of a dog's mask is dependent on other DNA variants such as the B and D loci that modify the production or appearance of the eumelanin (black pigment) in the dog.

For example, dogs that are b/b at the B locus will have brown masks due to the disruption of eumelanin synthesis that results in the conversion of black pigmented areas to brown.