CD9 is a tetraspanin membrane protein that organizes cell surface molecular complexes and regulates multiple cellular processes. CD9 spans the membrane four times and facilitates lateral organization of protein complexes in tetraspanin-enriched microdomains 1. In reproduction, CD9 is critical for sperm-egg fusion, with Cd9(-/-) mice being infertile due to defective sperm-egg interaction 2. The protein regulates cellular invasion processes, including extravillous trophoblast invasion during placental formation, where CD9 knockdown enhances invasion 3. CD9 also facilitates pathogen invasion, as both CD81 and CD9 are recruited to Salmonella entry sites during epithelial cell invasion 4. In immune cells, CD9 colocalizes with MHC Class II molecules in detergent-resistant membrane microdomains of eosinophils, supporting antigen presentation function 5. CD9 has anti-inflammatory properties, with CD9-enriched microdomains negatively regulating LPS-induced inflammation by preventing CD14 accumulation in lipid rafts 1. The protein also influences extracellular vesicle composition, with aging reducing CD9 expression in fibroblasts and altering EV cargo to promote angiogenesis 6. However, CD9 plays only a marginal role in determining overall EV protein composition 7. The CD9 gene spans over 20 kb with 8 exons and lacks TATA/CAAT boxes but contains GC-rich regulatory sequences 8.