CD133 (Promenin 1): A pentaspan transmembrane glycoprotein domain expressed in several stem cell populations and cancers. Possible role in the organization of plasma membrane topology.Expressed on CD34+ stem and progenitor cells in fetal liver, endothelial precursors, fetal neural stem cells, and developing epithelium. CD133 has been detected by its glycosylated epitope in the majority of studies. Thus, CD133 may be a more reliable cancer stem cell marker. The gene discussed is CD34; the disease is cancer.