Phosphaturic mesenchymal tumours are by far the most common tumour to cause tumour-induced osteomalacia (TIO), a rare paraneoplastic syndrome due to tumoural release of fibroblast growth factor (FGF)-23 causing phosphate wasting; clinically presenting with fatigue, bone pain, fractures, and a resistance to vitamin D supplementation. The gene discussed is FGF23; the disease is neoplasm.