After continuous stimulation by chemotherapeutic drugs, tumor cells will upregulate drug efflux–related transporters, especially including multidrug resistance protein 1 (MDR1), multidrug resistance–associated protein 1 (MRP1), and breast cancer resistance protein (BCRP) (Shapira et al., 2011; Housman et al., 2014). The gene discussed is ABCG2; the disease is neoplasm.