Since intrinsic resistance was also observed in BCRP/ABCG2-negative cancer cells (Figs. 4A and B), the BCRP/ABCG2-mediated drug efflux may not be the only mechanism contributing to insensitivity of wtEGFR-expressing cancer cells to gefitinib, and other mechanisms await to be explored. The gene discussed is ABCG2; the disease is cancer.