ABCG2 and small cell lung carcinoma: Other ABC transporters, including ATP-binding cassette sub-family B member 1 (ABCB1), a P-glycoprotein (P-gp), have been linked to resistance against etoposide (a topoisomerase II inhibitor), while ATP-binding cassette sub-family G member 2 (ABCG2) has been associated with resistance to SN-38, the active metabolite of irinotecan (a topoisomerase I inhibitor) in SCLC cell lines.