ABCB1 and glioblastoma: Interestingly, we do not observe double minute chromosomes at any step of the selection, in contrast with what has been shown for Abcb1 (P-glycoprotein) in colchicine-resistant J774.2 cells [30], or for ABCG2 in mitoxantrone-resistant glioblastoma cells [31], which display double minutes at low drug concentrations, but hsr at high concentrations.