Responsible for the deacetylation of lysine residues on the N-terminal part of the core histones (H2A, H2B, H3 and H4). Histone deacetylation gives a tag for epigenetic repression and plays an important role in transcriptional regulation, cell cycle progression and developmental events. Represses MEF2-dependent transcription. Isoform 3 lacks active site residues and therefore is catalytically inactive. Represses MEF2-dependent transcription by recruiting HDAC1 and/or HDAC3. Seems to inhibit skeletal myogenesis and to be involved in heart development. Protects neurons from apoptosis, both by inhibiting JUN phosphorylation by MAPK10 and by repressing JUN transcription via HDAC1 recruitment to JUN promoter. H2O + N(6)-acetyl-L-lysyl-[histone] = acetate + L-lysyl-[histone] Inhibited by Trichostatin A (TSA) and suberoylanilide hydroxamic acid. Homodimer. Interacts with CTBP1. The phosphorylated form interacts with 14-3-3 (By similarity). Interacts with HDAC1 and HDAC3, and probably with HDAC4 and HDAC5. Interacts with MEF2, MAPK10, ETV6, NCOR1 and BCL6. Interacts with FOXP3 in the absence of T-cell stimulation. Additional isoforms seem to exist. Broadly expressed, with highest levels in brain, heart, muscle and testis. Isoform 3 is present in human bladder carcinoma cells (at protein level). Phosphorylated on Ser-220 and Ser-450; which promotes 14-3-3-binding, impairs interaction with MEF2, and antagonizes antimyogenic activity. Phosphorylated on Ser-240; which impairs nuclear accumulation (By similarity). Isoform 7 is phosphorylated on Tyr-1010. Phosphorylated by the PKC kinases PKN1 and PKN2, impairing nuclear import. Sumoylated. A chromosomal aberration involving HDAC9 is found in a family with Peters anomaly. Translocation t(1;7)(q41;p21) with TGFB2 resulting in lack of HDAC9 protein. Major form in most tissues. Inactive due to lack of active site residues. Excluded from the nucleus. Does not interact with ETV6. Belongs to the histone deacetylase family. HD type 2 subfamily.