Among them, the phosphate-regulating gene homologous to endopeptidase on X chromosome (PHEX), whose product is a member of the M13 family of type II cell-surface zinc-dependent proteases, is responsible for X-linked hypophosphatemia (XLH; OMIM#307800), the most common form of hereditary hypophosphatemic rickets [15], [16]. The gene discussed is PHEX; the disease is hypophosphatemic rickets.