ABCG2 and brain neoplasm: Chemotherapy of primary and secondary brain tumors is often unsuccessful, in part because ABCB1 and ABCG2 at the blood-brain barrier restrict access to a wide range of anticancer drugs to the brain and impair their efficacy (de Vries et al., 2012; Taskar et al., 2012; de Gooijer et al., 2018d; Sorf et al., 2018).