While a considerable number of preclinical studies report the use of HDAC inhibitors [15] and underly the importance of HDACs’ deregulation in GBM malignancy, few clinical trials (22 on 10 September 2024) employed or are employing HDACi, usually as a combo-therapy with TMZ and radiations, suggesting that the use of these small molecules, also called epidrugs, still has to be implemented, and further studies are needed to identify the best therapeutic candidate. This evidence concerns the gene HDAC9 and glioblastoma.