ABCG2 and glioblastoma: Despite the fact that resistance to TMZ was associated with the abundant expression of O6-methylguanine-DNA-methyltransferase in CD133 positive GBM stem cells (Liu et al., 2006), a major cause of chemoresistance in GSCs was the activation of multi-drug resistance ATP-binding cassette (ABC) transporter genes (Dean et al., 2005), which had higher levels of expression in CSCs than their differentiated counterparts (Nakai et al., 2009).