ABCG2 and brain disorder: Complementing these uptake systems, the BBB expresses a range of highly effective ATP-binding cassette (ABC) family efflux transporter systems, most notably P-glycoprotein, breast cancer resistance protein (BCRP), multidrug resistance-associated protein (MRP-1 and MRP-2), which together serve to limit exposure of the CNS to potentially neurotoxic molecules [12], and which unfortunately are also a major barrier to the therapeutic treatment of brain diseases with pharmacological agents [13].