The data above revealed that dacomitinib significantly increased the sensitivity of MDR cancer cells with overexpressing ABCG2 to conventional substrate chemotherapeutic agents of ABCG2 in vitro and in vivo. As shown in Fig. 3, the intracellular accumulation of ABCG2 substrates DOX or Rho 123 in the resistant S1-MI-80 cells were remarkably lower than that in the parental S1 cells. This evidence concerns the gene ABCG2 and cancer.