CD9, also known as CD9 molecule, is a transmembrane protein belonging to the tetraspanin family. It functions as a cell surface glycoprotein with four transmembrane regions and two extracellular loops containing conserved disulfide bonds found in the tetraspanin family. CD9 plays a crucial role in various biological processes, including adhesion, motility, membrane fusion, signaling, and protein trafficking. Moreover, CD9 is implicated in platelet activation and aggregation, muscle cell fusion, and myotube maintenance. Overall, CD9's multifaceted functions make it a significant protein with implications in various biological processes and medical applications.