Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome characterized by fibroblast growth factor-23 (FGF-23) overproduction from mesenchymal tumors, leading to renal phosphate wasting, hypophosphatemia, and impaired 1,25-dihydroxyvitamin D (1,25 Vit D) synthesis. The gene discussed is FGF23; the disease is mesenchymal cell neoplasm.