ABCB1 and brain neoplasm: However, due to its large size (MW 825 Daltons) and susceptibility to transport by ATP-binding cassette (ABC) drug efflux transporters such as P-glycoprotein (P-gp, encoded by the gene MDR-1/ABCB1 gene) and multidrug resistance protein (MRP1, encoded by the MRP1/ABCC1 gene) [54–56], vincristine does not penetrate well into brain tumour tissue.