Blood examination revealed a low serum phosphorus level of 2.3 mg/dL (reference range: 3.0-4.7 mg/dL), and a markedly high serum FGF23 level of 329 pg/mL (reference range: <50 pg/mL), and so we suspected PMT with TIO caused by the tumor-like lesion in the left fibula. The gene discussed is FGF23; the disease is neoplasm.