At low concentrations, increased local secretion of FGF23 by osteocytes may promote osteoblast differentiation and bone formation by acting diffusely on neighbouring cells, whereas at a high concentration, systemic overproduction of FGF23 enters the circulation and acts on target organs such as the kidney, inhibiting renal phosphorus reabsorption and leading to hypophosphatasia rickets. The gene discussed is FGF23; the disease is hypophosphatasia.