Tumor-induced osteomalacia (TIO), also referred to as oncogenic osteomalacia (OO) or oncogenic hypophosphatemic osteomalacia (OHO), is a rare paraneoplastic syndrome caused by overproduction of fibroblast growth factor 23 (FGF-23) by small, usually benign, mesenchymal tumors originating in soft tissues or bone. The gene discussed is FGF23; the disease is mesenchymal cell neoplasm.