The molecule is a 5-transmembrane cell surface antigen with a molecular weight of 117 kDa. The CD133/1 (clone AC133) antibody recognizes epitope 1 of the CD133 antigen. In contrast, CD133/2 (clone 293C3) recognizes a different epitope on the CD133 antigen.

CD133 is expressed on hematopoietic stem cells, circulating endothelial progenitor cells, and fetal neural stem cells as well as on other tissue-specific stem cells, such as renal, prostate, and corneal stem cells. In addition, CD133 was found to be specifically expressed on cancer stem cells in multiple tumor entities such as glioblastoma, lung cancer, prostate cancer, pancreatic cancer, and renal cancer. In contrast to the hematopoietic system, where epitope CD133/1 and CD133/2 are co-expressed, only CD133/1 is expressed in most of the analyzed tumor entities. Therefore, it is crucial to use CD133/1 (clone AC133) for cell isolation.