ABCB1 and glioblastoma: Although ABCG2 is higher in GBM6R, available data suggest it is unlikely to explain the phenotype: in GBM cells ABCB1 dominates intracellular TMZ efflux, while ABCB1/ABCG2 chiefly restrict brain entry at the BBB—an exposure effect that cannot account for within-tumor GBM6 vs GBM6R differences [21, 22].