CD9 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. CD9 is commonly used as a marker for exosomes as it is contained on their surface. 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[24] and migration.[25][26] This function makes CD9 of interest when studying cancer and cancer metastasis. However, it seems CD9 has a varying role in different types of cancers. Studies showed that CD9 expression levels have an inverse correlation to metastatic potential or patient survival.

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