TIO, also known as oncogenic osteomalacia, is a rare paraneoplastic syndrome characterized by excess production by tumors of FGF-23 and rarely other phosphatonins such as frizzled-related protein-4, fibroblast growth factor-7 (FGF-7), and matrix extracellular phosphoglycoprotein (MEPE), leading to hypophosphatemia, muscle weakness, and fractures (5–7). The gene discussed is MEPE; the disease is hypophosphatemia.