Sclerostin (SOST) is a member of the cerberus/DAN family, a group of secreted glycoproteins characterized by a cysteine-knot motif. Cerberus/DAN family members are putative BMP antagonists, and include Dan, Cerberus, Gremlin, PRDC, and Caronte. While the overall sequence identity between members of the family is low, they have conserved spacing of six cysteine residues. Cerberus and Dan have an additional cysteine residue used for dimerization? however, SOST does not and is secreted as a monomer. SOST was originally identified as an important regulator of bone homeostasis. SOST is expressed by osteoclasts in developing bones of mouse embryos, including both intramembranously forming skull bones and endochondrally forming long bones. SOST plays a physiological role as a negative regulator of bone formation by repressing BMP-induced osteogenesis. SOST has been shown to have unique ligand specificity, binding BMP-5, -6, and -7 with high affinity and BMP-2 and -4 with low affinity.