Periostin (POSTN,PN),originally named as osteoblast-specific factor 2 (OSF-2),is a 90-kDa secreted protein which is now classified as a matricellular protein. It is present in a wide variety of normal adult tissues and fetal tissues,and has a role in bone,tooth and heart development and function. Studies show that periostin is overexpressed in a broad range of human cancer types,includidng lung,ovary,breast and colon cancers. Recent evidence reveals that periostin is expressed by fibroblasts in the normal tissue and in the stroma of the primary tumour,and it is required to allow cancer stem cell maintenance.