ATP-binding cassette subfamily G member 2 (ABCG2), a member of the ABC transporter superfamily proteins. the expression of ABCG2 is related to tumorigenic cancer stem cells (CSC) in several cancers, mediates multidrug resistance (MDR) by transporting substrate anticancer drugs out of cancer cells and decreasing their intracellular accumulation. ABCB1 and ABCG2 are efflux transporters which have a major impact on the pharmacological behavior of numerous drugs. They are expressed in the intestine, liver, kidney, BBB and placenta. It has become evident that ABCB1 and ABCG2 modify the pharmaco/toxicokinetics in the placenta and fetus and may consequently affect the outcome of pregnancy.