CD9 is a gene encoding a protein that is a member of the transmembrane 4 superfamily also known as the tetraspanin family. It is a cell surface glycoprotein that consists of four transmembrane regions and has two extracellular loops that contain disulfide bonds which are conserved throughout the tetraspanin family. Also containing distinct palmitoylation sites that allows CD9 to interact with lipids and other proteins. Tetraspanin proteins are involved in a multitude of biological processes such as adhesion, motility, membrane fusion, signaling and protein trafficking. Tetraspanins play a role in many biological processes because of their ability to interact with many different proteins including interactions between each other. Their distinct palmitoylation sites allow them to organize on the membrane into tetraspanin-enriched microdomains (TEM). These TEMs are thought to play a role in many cellular processes including exosome biogenesis. CD9 is commonly used as a marker for exosomes as it is contained on their surface.