CD9 is a member of the tetraspanin family, a type of cell surface glycoprotein characterized by four transmembrane domains and two extracellular loops with conserved disulfide bonds. It plays a role in various cellular processes, including cell adhesion, signal transduction, and the suppression of cancer cell motility and metastasis. CD9 is also involved in immune responses, such as the activation of the PI3K/AKT signaling pathway in B-lineage acute lymphoblastic leukemia. Additionally, it has been implicated in the regulation of extracellular vesicle secretion and is a potential biomarker for several cancers.