In pathophysiological situations with a primary surplus of FGF23, such as the genetic diseases autosomal dominant hypophosphatemic rickets (ADHR), autosomal recessive hypophosphatemic rickets (ARHR) and X-linked hypophosphatemia (XLH), and tumor-induced osteomalacia (TIO), hypophosphatemia occurs, which may lead to rickets or osteomalacia [19]. The gene discussed is FGF23; the disease is autosomal recessive hypophosphatemic rickets.