– Collagen α-1(XXI) Chain (COL21A1) is a member of the fibril-associated collagens with interrupted helices (FACIT) family. COL21A1 is a secreted protein and contains six collagen-like domains, one TSP N-terminal (TSPN) domain, and one VWFA domain. COL21A1 is widely expressed in many tissues with the highest expression observed at the fetal stage. COL21A1 is stimulated by PDGF/platelet-derived growth factor. Type XXI collagen is localized to tissues containing type I collagen; it may serve to maintain the integrity of the extracellular matrix.