The mammalian hair follicle (HF) is an active skin appendage which operates hair cycles throughout life. Recent advances in molecular genetics have led to the identification of many genes expressed in the HF. Furthermore, mutations in some of these genes have been shown to underlie congenital hair loss disorders in humans. Patients with congenital hair loss disorders can show various hair shaft anomalies, such as woolly hair and monilethrix. In the Japanese populations, most patients with congenital woolly hair/hypotrichosis possess common founder mutations in the lipase H (LIPH) gene. Identification of the causative genes for hair loss disorders directly demonstrates crucial roles of these genes in HF morphogenesis, development and/or hair growth in humans.