PROM1 and neoplasm: The in vivo effect of MEK/ERK inhibition was also assayed by treating mice bearing xenografts induced by rhabdosphere cells (25 % CD133 positive cells) (Fig. 5a) with an intraperitoneal injection of 25 μmoles/kg of U0126 when the tumor mass reached a volume of between 80 and 100 mm3.