Secondly, because cancer SP cells highly express three ATP-binding cassette (ABC) transporters, ABCB1 (also known as MDR1/P-glycoprotein), ABCC1, and ABCG2 (also known as BCRP) [30], they are thought to be chemo-resistant tumor-initiating cells [30]. The gene discussed is ABCG2; the disease is cancer.