The similarity of dependence on HDAC and KDM5A activity for AZD1775 resistance in acute leukemia cells, gefitinib resistance in NSCLC cells (8), and temozolomide resistance in glioblastoma cells (9), suggests multiple cancer types may adopt an altered epigenetic landscape that promotes drug tolerance in response to agents with differing mechanisms of action. This evidence concerns the gene HDAC9 and glioblastoma.