Gene defects underlying CHH concern genes encoding for proteins involved in different fundamental physiological mechanisms: development and migration of GnRH neurons as in the case of ANOS1, FGFR1, FGF8, CHD7, PROK2, and PROKR2; the regulation of GnRH secretion as in the case of KISS1R, TACR3 and TAC3; GnRH action as in the case of GNRHR. This evidence concerns the gene CHD7 and cartilage-hair hypoplasia.