CD133 (Prominin 1) is a penta-transmembrane surface glycoprotein coded by the gene PROM1. Following two papers reporting CD133 as a marker for CSCs in colorectal cancer (O’Brien et al., 2007; Ricci-Vitiani et al., 2007), there was a surge of reports showing CD133 as a marker for CSCs of different origin. The gene discussed is PROM1; the disease is colorectal cancer.