PROM1 and neoplasm: CD133+, but not CD133− colon cancer cells are able to generate tumors of the same histotype when injected in nude mice; CD133+ colon cancer cells can be amplified in vitro as floating aggregates called tumor spheres, which can be maintained in vitro and are able to form colon cancers when injected into immunodeficient SCID mice; and upon growth factor deprivation, CD133+ cells gradually differentiate, become adherent and express intestinal differentiation markers, such as CK20 [357,358].