For some tumor types (e.g., brain, liver, and Ewing sarcoma) CD133+ cell populations have been reported to be more tumorigenic than CD133− cell populations in xenograft assays (Singh et al., 2004; Jiang et al., 2010; Tang et al., 2011; von Levetzow et al., 2011). The gene discussed is PROM1; the disease is Ewing sarcoma.