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.

[18] It forms a alphaIIbbeta3-CD9-CD63 complex on the surface of platelets that interacts directly with other cells such as neutrophils which may assist in immune response.

[11][19] In addition, the protein appears to promote muscle cell fusion and support myotube maintenance.

Studies showed that CD9 expression levels have an inverse correlation to metastatic potential or patient survival.

The over expression of CD9 was shown to decrease metastasis in certain types of melanoma, breast, lung, pancreas and colon carcinomas.