Periostin ( POSTN ), also known as OSF2 (osteoblast specific factor 2), is a heterofunctional secreted extracellular matrix (ECM) protein comprised of four fasciclin domains that promotes cellular adhesion and movement, as well as collagen fibrillogenesis. Postn is expressed in unique growth centers during embryonic development where it facilitates epithelial-mesenchymal transition (EMT) of select cell populations undergoing reorganization. In the adult, Postn expression is specifically induced in areas of tissue injury or areas with ongoing cellular re-organization. In the adult heart Postn is induced in the ventricles following myocardial infarction, pressure overload stimulation, or generalized cardiomyopathy. Although the detailed function of Postn is still unclear, Postn-integrin interaction is thought to be involved in tumor development. Postn is frequently overexpressed in various types of human cancers, stimulating metastatic growth by promoting cancer cell survival, invasion and angiogenesis, and can be a useful marker to predict the behavior of cancer.