The biological effects of HDAC inhibition via vorinostat are widespread, with HDAC inhibitors causing changes in 2–10% of all expressed genes.33 There is preclinical evidence that vorinostat has antitumor activity against malignant glioma cell lines in vitro and orthotopic xenografts in vivo.22,34,35 A phase II trial of vorinostat in patients with recurrent GBM20 also revealed that vorinostat was well tolerated, and paired tumor samples from baseline and post-vorinostat treatment revealed increased histone acetylation that was consistent with the expected targeted effect. Here, HDAC9 is linked to malignant glioma.