CD133 (also known as Prominin-1 or AC133) is a 120 kD pentaspan glycoprotein with 5 transmembrane domains which localize to cellular protrusions. CD133 was initially described as a surface antigen specific for human hematopoietic stem cells and as a marker for murine neuroepithelial cells and some embryonic epithelia. Later on, CD133 was found on other stem cells including endothelial progenitor cells, glioblastomas, neuronal, and glial stem cells. In addition to hematopoietic stem cells, CD133 was found on cancer cells, such as some leukemia cells and brain tumor cells. It also has been detected at low levels in the kidney, pancreas, placenta, and fetal liver tissue. Although the biological function of CD133 is not conclusive, CD133 has been extensively used as a stem cell marker for normal and cancerous tissues.