The resulting excess circulating levels of fibroblast growth factor 23 (FGF23) lead to renal phosphate wasting and hypophosphatemia,1 with deleterious effects on bone growth and quality (bone turnover, microarchitecture, and mineralization), abnormalities in muscle structure and function, and impaired dental mineralization.1–4. The gene discussed is FGF23; the disease is hypophosphatemia.