Mice homozygous for the hr mutation have a higher incidence and earlier onset of leukemia, reducible by virus-specific antibody. Deficiency of splenic T helper cells (Ly-1+) may account for low cellular immune response of homozygous mutant mice. The coat is normal on hr/hr mice up to 10 days but then hair is lost from the follicle. Waves of hair growth with few thin fuzzy hairs ocur at monthly intervals for some time but homozygotes eventually become continuously hairless. Vibrissae are repeatedly regrown and shed, becoming more abnormal with age. Toenails are long and curved. There is hyperkeratosis of statified epithelium and the upper part of hair canals beginning at 14 days. Hair club formation is abnormal. Cysts form from the hyperkeratotic upper part of hair canals and sheaths of abnormal follicles stranded in dermis. Some cysts also form from sebaceous glands. All cysts undergo sebaceous transformation and later keratinization.