Osteoblast-Specific Factor OSF-2 (POSTN, Periostin, OSF-2) is a secreted, homodimeric protein that belongs to the periostin family of the FAS1 superfamily of molecules. Periostin is a disulfide linked 90kDa bone adhesion protein secreted by osteoblasts and osteoblast-like cell lines and the protein is an attachment agent for osteoblasts. It is a TGF-beta inducible molecule that serves as both an adhesion molecule and tumor suppressor. It is synthesized by smooth muscle cells, fibroblasts and osteoblasts, as well as in the periosteum and periodontal ligament. Periostin functions as a ligand for alpha-V/beta-3 and alpha-V/beta-5 integrins to support adhesion and migration of epithelial cells.