The Hairless nuclear receptor co-repressor is required for hair follicle regeneration during the hair cycle. The classical HairlessHr/HairlessHr mouse mutant loses all hair between 2 and 3 weeks of age. As the mice age, their trunk skin develops epidermal pigmentation, a feature of human skin which is not found in normal haired mice. In this report, we present a new, dominant mouse mutation, Pied, which arose within a colony of HairlessHr/HairlessHr mice and causes freckle-like macules on the skin. The Pied macules require HairlessHr homozygosity to form and are composed of localized clusters of epidermal melanocytes. Through linkage analysis, we find that the Pied mutation is a 1914 base pair loss-of-function deletion in the Adam10 zinc metalloprotease gene. The pathways that specifically maintain long-term pigmentation patterns in adults are not well understood. We have identified Adam10 as an inhibitor of melanocyte expansion in adult skin.