PGP and neoplasm: Comparison of these subpopulations suggests that CD133+ cells have 1) increased level of functional Pgp efflux system (as followed from exclusion of a Pgp substrate, Fig. 6a), 2) increased resistance with respect to Dox (Table 3), 3) increased colony formation potential in vitro (Fig. 6b) and 4) increased tumorigenicity in vivo (as assayed by tumor frequency formation after s.c. inoculation of cells in mice, Table 4).