When low concentrations of QUE were used in combination with DOX, QUE increased the accumulation of DOX in MCF-7 and MDA-MB-231 cells by down-regulating the expression of ABCB1, BCRP, and MRP1 and promoted the killing effect of DOX on BC cells, especially BC stem cells (77). The gene discussed is ABCG2; the disease is breast cancer.