Integral membrane protein associated with integrins, which regulates different processes, such as sperm-egg fusion, platelet activation and aggregation, and cell adhesion (By similarity). 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 (By similarity). In myoblasts, associates with CD81 and PTGFRN and inhibits myotube fusion during muscle regeneration (By similarity). In macrophages, associates with CD81 and beta-1 and beta-2 integrins, and prevents macrophage fusion into multinucleated giant cells specialized in ingesting complement-opsonized large particles (By similarity). Also prevents the fusion between mononuclear cell progenitors into osteoclasts in charge of bone resorption (By similarity). Acts as a receptor for PSG17 (By similarity). Involved in platelet activation and aggregation (By similarity). Regulates paranodal junction formation (By similarity). Involved in cell adhesion, cell motility and tumor metastasis (By similarity). Forms both disulfide-linked homodimers and higher homooligomers as well as heterooligomers with other members of the tetraspanin family. 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 (By similarity). 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. Associates with CR2/CD21 and with PTGFRN/CD9P1. Part of a complex composed of CD9, CD81, PTGFRN and IGSF8 (By similarity). Interacts directly with IGSF8. Interacts with PDPN; this interaction is homophilic and attenuates platelet aggregation and pulmonary metastasis induced by PDPN. Interacts (on T cell side) with CD81 at immunological synapses between antigen-presenting cells and T cells (By similarity). Present at the cell surface of oocytes. Accumulates in the adhesion area between the sperm and egg following interaction between IZUMO1 and its receptor IZUMO1R/JUNO. Ubiquitous. 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. Belongs to the tetraspanin (TM4SF) family.