Histone deacetylase that catalyzes the deacetylation of lysine residues on the N-terminal part of the core histones (H2A, H2B, H3 and H4) (PubMed:18754010). Histone deacetylation gives a tag for epigenetic repression and plays an important role in transcriptional regulation, cell cycle progression and developmental events (PubMed:18754010). Histone deacetylases act via the formation of large multiprotein complexes (PubMed:18754010). Forms transcriptional repressor complexes by associating with MAD, SIN3, YY1 and N-COR (By similarity). Component of a RCOR/GFI/KDM1A/HDAC complex that suppresses, via histone deacetylase (HDAC) recruitment, a number of genes implicated in multilineage blood cell development (PubMed:17707228). Also deacetylates non-histone targets: deacetylates TSHZ3, thereby regulating its transcriptional repressor activity (By similarity). May be involved in the transcriptional repression of circadian target genes, such as PER1, mediated by CRY1 through histone deacetylation (PubMed:15226430). Involved in MTA1-mediated transcriptional corepression of TFF1 and CDKN1A (PubMed:20071335). In addition to protein deacetylase activity, also acts as protein-lysine deacylase by recognizing other acyl groups: catalyzes removal of (2E)-butenoyl (crotonyl) and 2-hydroxyisobutanoyl (2-hydroxyisobutyryl) acyl groups from lysine residues, leading to protein decrotonylation and de-2-hydroxyisobutyrylation, respectively (PubMed:30279482). H2O + N(6)-acetyl-L-lysyl-[histone] = acetate + L-lysyl-[histone] H2O + N(6)-acetyl-L-lysyl-[protein] = acetate + L-lysyl-[protein] H2O + N(6)-(2E)-butenoyl-L-lysyl-[protein] = (2E)-2-butenoate + L-lysyl-[protein] H2O + N(6)-(2-hydroxyisobutanoyl)-L-lysyl-[protein] = 2-hydroxy-2-methylpropanoate + L-lysyl-[protein] Part of the core histone deacetylase (HDAC) complex composed of HDAC1, HDAC2, RBBP4 and RBBP7 (By similarity). The core complex associates with MTA2, MBD3, MTA1L1, CHD3 and CHD4 to form the nucleosome remodeling and histone deacetylation (NuRD) complex, or with SIN3, SAP18 and SAP30 to form the SIN3 HDAC complex (By similarity). Component of a RCOR/GFI/KDM1A/HDAC complex (PubMed:17707228). Component of a BHC histone deacetylase complex that contains HDAC1, HDAC2, HMG20B, KDM1A, RCOR1 and PHF21A (By similarity). The BHC complex may also contain ZMYM2, ZNF217, ZMYM3, GSE1 and GTF2I (By similarity). Part of a complex containing the core histones H2A, H2B, H3 and H4, DEK and unphosphorylated DAXX (By similarity). Part of a complex containing ATR and CHD4 (By similarity). Forms a heterologous complex at least with YY1 (By similarity). Interacts in the late S-phase of DNA-replication with DNMT1 in the other transcriptional repressor complex composed of DNMT1, DMAP1, PCNA, CAF1 (By similarity). Component of a mSin3A corepressor complex that contains SIN3A, SAP130, SUDS3, ARID4B, HDAC1 and HDAC2 (By similarity). Part of a complex composed of TRIM28, HDAC1, HDAC2 and EHMT2 (By similarity). Part of a complex containing at least CDYL, MIER1, MIER2, HDAC1 and HDAC2 (By similarity). Component of a histone deacetylase complex containing DNTTIP1, ZNF541, HDAC1 and HDAC2 (By similarity). Forms a complex comprising APPL1, RUVBL2, APPL2, CTNNB1 and HDAC1 (By similarity). Interacts with SPHK2 (By similarity). Interacts directly with GFI1 and GFI1B (PubMed:17707228). Interacts with SNW1, HDAC7, PRDM6, SAP30, SETDB1 and SUV39H1 (PubMed:11788710, PubMed:12398767, PubMed:10984530, PubMed:9702189, PubMed:16537907). Interacts with the MACROH2A1 (via the non-histone region) (PubMed:16107708). Interacts with ATR, CBFA2T3, DNMT1, SMARCAD1, MINT, HDAC10, HCFC1, NRIP1, KDM4A and PELP1 (PubMed:11533236). Interacts with CHFR and SAP30L (By similarity). Interacts (CK2 phosphorylated form) with SP3 (By similarity). Interacts with TSHZ3 (via its N-terminus) (By similarity). Interacts with APEX1; the interaction is not dependent on the acetylated status of APEX1 (By similarity). Interacts with PIMREG (By similarity). Interacts with BCL6 (non-acetylated form) (By similarity). Interacts with CRY1, INSM1 and ZNF431 (PubMed:21177534, PubMed:22391310, PubMed:24227653, PubMed:15226430). Interacts with NACC2 (By similarity). Interacts with MTA1, with a preference for sumoylated MTA1 (By similarity). Interacts with SIX3 (PubMed:17666527). Interacts with BEND3 (By similarity). Interacts with ZNF263; recruited to the SIX3 promoter along with other proteins involved in chromatin modification and transcriptional corepression where it contributes to transcriptional repression (By similarity). Interacts with PWWP2B (PubMed:34180153). S-nitrosylated by GAPDH. In neurons, S-nitrosylation at Cys-262 and Cys-274 does not affect enzyme activity, but induces HDAC2 release from chromatin. This in turn increases acetylation of histones surrounding neurotrophin-dependent gene promoters and promotes their transcription. In embryonic cortical neurons, S-Nitrosylation regulates dendritic growth and branching. Belongs to the histone deacetylase family. HD type 1 subfamily. Was originally thought to be S-nitrosylated and to interact with MTA1 (PubMed:20519513). However, this work was later retracted (PubMed:28314777). Nevertheless, other publications demonstrate that it is S-nitrosylated and there are several publications in the human ortholog demonstrating its interaction with MTA1 (PubMed:18754010, PubMed:20972425).