ABCB1 and cancer: An over-abundance of P-glycoprotein (P-gp), the protein product of the MDR1 gene [1], leads to multidrug resistance (MDR), because cancer cells become able to efflux a number of structurally diverse compounds including many chemotherapy agents, such as paclitaxel [11], [12], doxorubicin [13], and vinblastine [14].