FGF23 and X-linked hypophosphatemia: In addition to the well characterized role of elevated FGF23 and hypophosphatemia in XLH, inactivating PHEX variants also modify levels of the small integrin-binding ligand, N-linked glycoproteins (SIBLING) family, such as osteopontin and the inhibitory acidic serine aspartate-rich-MEPE-associated protein (ASARM) peptides (1, 15–17).