Since BCRP/ABCG2-positive tumors were found in 46% of advanced NSCLC patients [33], the BCRP/ABCG2-dependent drug resistance to gefitinib but not erlotinib might explain why erlotinib provides a better clinical outcome than gefitinib [7], [8] and may serve as a salvage treatment for NSCLC patients after failure of gefitinib treatment [34], [35]. Here, ABCG2 is linked to non-small cell lung carcinoma.