ABCG2 and cancer: Increasing the active pump-out ability of anti-tumor drugs and reducing the concentration of the drugs in cells are known to be important ways for cancer cells to develop drug resistance, similar to the ABC transporter family member P-glycoprotein encoded by MDR1 and MDR2 acting as a pump to limit drug accumulation in cells to achieve drug resistance (Wu & Ambudkar, 2014; Xue & Liang, 2012; Nieth et al., 2003; Abdallah et al., 2016).