Poliosis, also called poliosis circumscripta, is the decrease or absence of melanin (or colour) in head hair, eyebrows, eyelashes or any other hairy area.
This condition can cause single or, less commonly, multiple white patches on the hair. Some mistake these white patches for simple birth marks. In poliosis there is decreased or absent melanin in the hair bulbs of affected hair follicles; the melanocytes of the skin are usually not affected. Poliosis occurs in several genetic syndromes such as piebaldism, Waardenburg syndrome, neurofibromatosis type I, and tuberous sclerosis. It can also occur in conditions such as vitiligo, Vogt–Koyanagi–Harada disease, alopecia areata, sarcoidosis, and in association with neoplasms and some medications.
It can give rise to a "Mallen Streak" that can be hereditary. Catherine Cookson wrote a novel and later a TV series called The Mallen Streak, where a family had the condition. Sports presenter Dickie Davies and runner Sam Brown were famous for this feature.