||CD9 is expressed on platelets, eosinophils, basophils, pre-B cells, activated T cells and neural cell lines. It belongs to a member of the tetraspanin transmembrane-protein (TM4) superfamily, which includes CD37, CD53, CD63, CD81, CD82, CD151 and CD231. Several members of this family, including CD9, form noncovalent associations with integrins, particularly b1 integrins (CD29). Tetraspanins are involved in membrane events such as sperm-oocyte fusion, myoblast fusion, mononuclear phagocyte fusion, osteoclastogenesis, and paranodal junction formation in the peripheral nervous system. In addition, tetraspanins are implicated in viral processes such as CD63 in HIV infection, CD81 in hepatitis C virus infection, CD82 in cell-to-cell human T cell leukemia virus type I (HTLV-1) spreading, and CD9 in feline immunodeficiency virus (FIV) and canine distemper virus spreading.