ABCG2 and neoplasm: As shown in Supplementary Figure S2, mRNAs encoding ABCG2 and ABCB1 were more abundant in CD44+CD133+ tumor-initiating Caco-2 cells than in ΔCD44+CD133+ non-tumor-initiating Caco-2 cells (i.e., the CD44−CD133- and CD44−CD133+ fractions).