CD133, originally known as AC133, is a glycoprotein also known as Prominin 1 (PROM1). Currently the function of CD133 is unknown. It is a member of pentaspan transmembrane glycoproteins (5-transmembrane,  5-TM), which specifically localize to cellular protrusions.

A compelling number of studies has shown that CD133 is a marker for cancer stem cells (CSCs) in a wide variety of tumors, including pancreatic adenocarcinoma, colon carcinoma, hepatocellular carcinoma and neural tumors. Thus, this molecule may serve as a promising novel target in cancer therapy.