ABCG2 and glioblastoma: Moreover, the SHH pathway is thought to be activated in primary GBM and glioma cell lines [148], leading to the overexpression of some drug efflux molecules (e.g., ABCB1, ABCG2 [BCRP], and ABCC1 [MRP1]), and MGMT and BMI1 [149,150], which may explain its association with therapy-resistance.