OPN is an acidic, phosphorylated, calcium-binding, extracellular matrix protein from the small integrin-binding ligand, N-linked glycoprotein (SIBLING) family (Fisher and Fedarko, 2003; Qin et al., 2004), and has been proposed as a key player in XLH pathobiology. The gene discussed is SPP1; the disease is X-linked hypophosphatemia.