FGF23 and hereditary hypophosphatemic rickets: Mutations in fibroblast growth factor 23 (FGF23) and PHEX account for approximately 60% of hypophosphatemic rickets with eucalciuria and are associated with abnormalities in dentin and bone.(1) Mutations in the type 2a sodium-phosphate cotransporter lead to hereditary hypophosphatemic rickets with hypercalciuria (HHRH).(2–6) The molecular defects responsible for the remaining hypophosphatemic patients are unknown.