ABCB1 and neoplasm: Across all pediatric and adult brain tumors, we found a subset of ATP-binding cassette (ABC) transporters, which play an important role in mediating efflux of xenobiotics, thereby regulating the permeability of the NVU [35], to be mostly underexpressed relative to normal brain tissue (ABCG4, ABCA2, ABCA10, ABCA5, ABCB1, ABCA7, ABCA9, ABCG2, ABCA3), while ABCA8 and ABCA6 showed variable patterns of expression across the set of tumor histologies (Figure 3A).