From National Geographic:
To learn how a gene called Hairless regulates hair growth, scientists studied a line of completely bald mice that lacks the Hairless gene. These mice start with a full coat of fur, but once it falls out it never grows back. By genetically engineering the hairless mice to produce Hairless protein in specific cells within their hair follicles, the scientists caused the mice to regrow thick fur. The hair growth cycle has several stages: growth, regression, rest, and reinitiation of growth. If something goes wrong with this process, hair thinning or baldness may result. After hair grows to a particular length, it falls out and the lower part of the follicle is destroyed. After a period of rest, however, the follicle receives a signal that tells it to regrow its lower part and produce a new hair. Until the new findings were made, the exact nature of that chemical signal remained unknown. Hairless “turns off” a gene that makes a protein called Wise. In cells lacking Hairless, continual accumulation of Wise appears to prevent the hair cycle from switching from the rest to the regrowth phase.
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