PROM1 and glioblastoma: While negative results are due in part to a poor penetration of the blood–brain barrier by several molecules, the presence of CD133-positive cells with an increased expression of the drug-transporter protein BCRP1 (breast cancer resistance protein 1) and MGMT, as well as anti-apoptosis proteins like Bcl-2 and Bcl-xL, and apoptosis inhibitors like FLIP (FLICE (caspase-8) inhibitory protein), cIAP1, and survivin could also play a role in the resistance to treatment in glioblastoma.