PROM1 and renal cell carcinoma: Although CD133+ cells from several human tumors exhibit xenotransplantation potential in severe combined immunodeficiency (SCID) mice [8, 18, 19, 29], CD133+ cells from human RCC tissue (RCCCD133+cells) [27] fail to form tumors when transplanted independently in SCID mice but instead potentiate tumor engraftment when co-transplanted with CD133− RCC cells [29].