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. CD9 has a diverse role in cellular processes as it has also been shown to trigger platelet activation and aggregation. CD9 can also modulate cell adhesion and migration. Additionally, CD9 has been shown to block adhesion of Staphylococcus aureus to wounds. The adhesion is essential for infection of the wound. This suggests that CD9 could be of possible use to as treatment for skin infection by Staphylococcus aureus.