Integral membrane protein associated with integrins, which regulates different processes, such as sperm-egg fusion, platelet activation and aggregation, and cell adhesion (PubMed:10700183, PubMed:10634790, PubMed:10634791, PubMed:14715942). Present at the cell surface of oocytes and plays a key role in sperm-egg fusion, possibly by organizing multiprotein complexes and the morphology of the membrane required for the fusion (PubMed:10700183, PubMed:10634790, PubMed:10634791, PubMed:21690351). In myoblasts, associates with CD81 and PTGFRN and inhibits myotube fusion during muscle regeneration (PubMed:23575678). In macrophages, associates with CD9 and beta-1 and beta-2 integrins, and prevents macrophage fusion into multinucleated giant cells specialized in ingesting complement-opsonized large particles. Also prevents the fusion between mononuclear cell progenitors into osteoclasts in charge of bone resorption (PubMed:12796480). Acts as a receptor for PSG17 (PubMed:11805154). Involved in platelet activation and aggregation (PubMed:14715942). Regulates paranodal junction formation (PubMed:14715942). Involved in cell adhesion, cell motility and tumor metastasis (By similarity). Also regulates integrin-dependent migration of macrophages, particularly relevant for inflammatory response in the lung (PubMed:18662991). Forms both disulfide-linked homodimers and higher homooligomers as well as heterooligomers with other members of the tetraspanin family (By similarity). Interacts (via the second extracellular domain) with integrin ITGAV:ITGB3 (By similarity). Interacts with integrin ITGA6:ITGB1; interaction takes place in oocytes and is involved in sperm-egg fusion (PubMed:10634791). Part of integrin-tetraspanin complexes composed of CD81, beta-1 and beta-2 integrins in the membrane of monocyte/macrophages (By similarity). Interacts with CD63; identified in a complex with CD63 and ITGB3 (By similarity). Associates with CR2/CD21 and with PTGFRN/CD9P1 (By similarity). Part of a complex composed of CD9, CD81, PTGFRN and IGSF8 (PubMed:23575678). Interacts directly with IGSF8 (By similarity). Interacts with PDPN; this interaction is homophilic and attenuates platelet aggregation and pulmonary metastasis induced by PDPN (By similarity). Present at the cell surface of oocytes (PubMed:10518536, PubMed:10634791, PubMed:23213457). Accumulates in the adhesion area between the sperm and egg following interaction between IZUMO1 and its receptor IZUMO1R/JUNO (PubMed:25209248). Expressed predominantly in the peripheral nervous system (PubMed:14715942). Highly expressed in oocytes and blastocysts (at protein level) (PubMed:10518536, PubMed:10634790, PubMed:10634791, PubMed:23213457). Expression is also observed on follicular oocytes in the ovary, whereas no expression is found on follicular cells (at protein level) (PubMed:10518536, PubMed:10634790). Expressed in skeletal muscle mainly in endothelial cells of endomysial capillaries, in satellite cells and myoblasts (at protein level). Palmitoylated at a low, basal level in unstimulated platelets. The level of palmitoylation increases when platelets are activated by thrombin (in vitro). The protein exists in three forms with molecular masses between 22 and 27 kDa, and is known to carry covalently linked fatty acids. Palmitoylation by ZDHHC2 regulates CD9 expression, association with other tetraspanin family proteins and function in cell adhesion. Knockout mice are healthy, but females display severely reduced fertility (PubMed:10634790, PubMed:10634791). Oocyte maturation and ovulation are normal and female infertility is caused by defects in sperm-egg fusion (PubMed:10634790, PubMed:10634791). Wild type sperm penetrates the zona pellucida, binds to the oolema but the membranes fail to fuse (PubMed:10634790, PubMed:10634791). The infertility is overcome by intracytoplasmic sperm injection, and embryos develop normally (PubMed:10634790, PubMed:10634791). Eggs show reduced ability for strong sperm adhesion, and sperm accumulate in the perivitelline space, only transiently binding to the egg surface (PubMed:21690351). In response to notexin-induced acute myoinjury, mutant mice display abnormal muscle regeneration characterized by typical giant distrophic myofibres (PubMed:23575678). These mice spontaneously develop multinucleated giant cells (MGCs) and show enhanced osteoclastogenesis when compared to wild-type littermates (PubMed:12796480). CD81 and CD9 double knockout mice develop pulmonary emphysema, reminiscent of chronic obstructive pulmonary disease in human (PubMed:18662991). Belongs to the tetraspanin (TM4SF) family.