It was first proposed that osteocytes are endocrine cells in 2006, and the first secreted factor identified was fibroblast growth factor 23 (FGF23), which is highly elevated in osteocytes of patients with hypophosphatemic rickets (Feng et al., 2006; Liu et al., 2006). The gene discussed is FGF23; the disease is hypophosphatemic rickets.