Monoclonal antibody K41 detects CD9, a member of the tetraspan or transmembrane 4 superfamily. The family members (CD9, CD63, CD37, Cd53, CD81, CD82, CD151) show a high degree of homology in their membrane-spanning domain. CD9 is the only member with N-linked sugar bound to the first extracellular segment. Tetraspans are expressed by many cell types and show a different subcellular distribution. CD9 is constitutively surface-expressed by platelets and pre-B cells and is present in the ?-granules of platelets. The participation of different tetraspan members in signaling complexes on B and T cells have been described. Furthermore, association of CD9 with ?1 integrins has been found. Anti CD9 monoclonal antibodies induce homotypic pre-B cell aggregation and fibronectin receptor-mediated adherence of these cells to bone marrow fibroblasts. Recent observations suggest a role of CD9 in adhesion. Application: IHC (f, p), FACS, Western Blot