CD133, known as AC133, is a member of a novel family of cell surface proteins that has five transmembrane domain molecules with an extracellular N-terminus and a cytoplasmic C-terminus, and has two large extracellular loops with eight consensus sites for N-linked glycosylation. It has two isoforms. AC133-1 mRNA was prominent in fetal brain and adult skeletal muscle but was not detected in fetal liver and kidney, adult pancreas, kidney, and placenta. AC133-2 mRNA was found to be dominant in a variety of human fetal tissues, adult tissues, and several carcinomas. CD133 has not only been used to isolate hematopoietic stem cells but also represent a marker of tumor initiating cells in a number of human cancers, such as in human pancreatic adenocarcinoma, colon cancer, melanoma, hepatocellular carcinoma, and brain tumors.
