ABCB1 and cancer: Regardless of MGMT status, other molecular mechanisms of drug resistance determining the limited response to TMZ treatment are also reported, such as ferroptosis [13] and the upregulation of MGMT-independent DNA repair mechanisms, such as Base Excision Repair (BER) [14], Multidrug Resistance Protein 1/P-glycoprotein (MDR1/Pgp) activity [15], and the presence of cancer stem cells [CSC] [16].