Although the brain parenchyma also displays a significant difference in drug concentrations in lesions and in normal brain tissue at the tumor periphery, the difference may be related to the integrity of the BTB and efflux pumps, such as P-glycoprotein (P-gp/ABCB1) and breast cancer resistance protein (BCRP/ABCG2) [19]. The gene discussed is ABCG2; the disease is neoplasm.