The nu gene bred into mice causes the animals to be athymic and nude, that is their hairs do not project past the follicular orifice making them appear to be hairless. The gene has been bred into C57BL/6 mice. C57BL/6 mice which are homozygous for the gene are runted and nude and develop leukoderma around 6 weeks of age. The leukoderma is caused by shedding of short stubby, barely visible black hairs, probably a normal hair cycle. This type of leukoderma is not vitiligo, a disease which causes the skin to turn white because of destruction of the melanocytes in the epidermis or hair bulbs.