CD151, also known as PETA-3, is a 29 kD protein member of the tetraspanin family (TM4SF). As other members in the family, PETA-3 is composed of two extracellular loops (one larger than the other), four transmembrane domains and two cytoplasmic tails. Also, as other members in the family, CD151 is involved in cell adhesion, migration, and tumor cell metastasis. PETA-3 is expressed by many different cell types such as immature haemopoietic cells, megakaryocytes, platelets, keratinocytes, epithelial cells, muscle cells, Schwann cells and vascular endothelium. In the cell surface PETA-3 associates with CD9, CD181, and the integrins α3beta1, α5beta1 and α6beta4.