CD9, also known as CD9 antigen, is a protein that in humans is encoded by the CD9 gene. CD9 is a cell surface glycoprotein that is known to complex with integrins and other transmembrane 4 superfamily proteins. It is found on the surface of exosomes. It can modulate cell adhesion and migration and also trigger platelet activation and aggregation. In addition, the protein appears to promote muscle cell fusion and support myotube maintenance. This protein also seems to be a key part in the egg-sperm fusion during mammalian fertilization, as CD9 knocked-out mice gametes don't undergo fusion. Recombinant human CD9, fused to His-tag at N-terminus, was expressed in E.coli and purified by using conventional chromatography techniques.