For example, inactivating mutations in the phosphate-regulating endopeptidase homolog, X-linked (PHEX) gene lead to the upregulation of FGF23 production in patients with XLH, which is the most prevalent form of genetic FGF23-related hypophosphatemic rickets (Liu et al., 2003). The gene discussed is PHEX; the disease is hypophosphatemic rickets.