Both CD133+ and CD133− metastatic cancer cells formed colonospheres and tumors in NOD/SCID mice that could be serially xenotransplanted, and it was noted the CD133− cells formed more aggressive tumors and were more enriched for phenotypic markers thought to be more typical of cancer initiating cells (CD44+CD24−) than CD133+ cells. The gene discussed is PROM1; the disease is cancer.