ABCG2 and cancer: Overexpression of ATP-binding cassette drug efflux transporters (ABC transporters, such as ABCB1 or MDR1) on the plasma membrane of cancer cell lines, isolated as paclitaxel- or olaparib-resistant cells, has been regarded as a mechanism of drug resistance toward relatively lipophilic anti-cancer drugs, such as doxorubicin, rucaparib, paclitaxel, and olaparib, for a long time[9].