HDAC4 and glioblastoma: HDAC1 expression is elevated in GBM tumor tissue as compared to normal surrounding brain tissue, and early gene expression profiling experiments in GBM revealed TCGA subtype exclusivity in the activity of histone acetyltransferase and deacetylase pathways, with proneural tumors having increased activation of HDAC1 and mesenchymal tumors having increased activation of HDAC4 and SIRT1 pathways (51–53, 55).