PROM1 and cancer: However, in colon cancers CD133+ and CD133− populations have been found to be equally capable of tumor initiation in xenografts (Shmelkov et al., 2008), and both cell fractions have substantial tumor initiating activity in melanoma, lung, and ovarian cancer (Meng et al., 2009; Shackleton, 2010; Stewart et al., 2011), making CD133 a controversial marker for cancer stem cells (LaBarge and Bissell, 2008).