ABCG2 and neoplasm: In addition to P-gp at the BBB, BCRP and other key efflux transporters such as MRP 1–5 that belong to the ABCC transporter family, play a critical role in restricting brain penetration of a large number of anti-tumor agents (Durmus et al., 2012; Lin, de Gooijer, et al., 2013; Gerber et al., 2014).