Hair growth is a complex process. One of the key roles in this process is played by the thyroid hormone, which is essential for the regeneration of hair follicles. The hairless gene codes for a nuclear receptor co-repressor, which functions by turning off the expression of thyroid receptors. Interestingly, the hairless gene itself is triggered by thyroid hormone, thereby maintaining a balance of the effects of thyroid hormone on the brain and skin. Thus, mutations in the hairless gene actually result in partial or total hair loss.
At least two separate forms of hair loss are associated with mutations in the HR gene. Alopecia universalis congenita (ALUNC) is an autosomal recessive condition characterized by hair follicles devoid of hair. On the other hand, atrichia with papular lesions (APL), also an autosomal recessive disorder, is characterized by total hair loss accompanied with the presence of papular lesions all over the skin.