CD97 antigen is a receptor involved in both adhesion and signaling processes early after leukocyte activation. It also plays an essential role in leukocyte migration. The unprocessed precursor is 853 amino acids long and has a predicted molecular weight of 91.9KDa. CD97 antigen is a heterodimer consisting of a large extracellular alpha subunit and a seven-transmembrane beta subunit that are non-covalently linked. It contains five EGF-like domains and one GPS domain. This protein interacts with complement decay-accelerating factor (DAF) through the first two EGF domains, and with chondroitin sulfate (only isoform 1) through the fourth EGF domain. CD97 antigen is broadly expressed and it is present on most hematopoietic cells. It is also abundantly expressed by smooth muscle cells. Expression of this protein is increased under inflammatory conditions in the CNS of multiple sclerosis and in synovial tissue of patients of rheumatoid arthritis. This protein is rapidly up-regulated during lymphocyte activation. C97 antigen belongs to the G-protein coupled receptor 2 fmaily and the LN-TM7 subfamily.