Class-I: Class I HDACs share homology with the yeast Rpd3. This class includes HDACs 1, 2, 3, and 8, and its members are expressed ubiquitously in humans. Members 1, 2, and 3 are found in multi-protein repressor complexes in the nucleus, where they regulate gene expression. HDAC8 is found in both the nucleus and cytoplasm, and while it has not been found in any multi-protein complexes, it has been described to be involved in transcriptional repression. (1)

Reference
1. Koeneke, E., Witt, O., & Oehme, I.HDAC Family Members Intertwined in the Regulation of Autophagy: A Druggable Vulnerability in Aggressive Tumor Entities. Cells.2015;4(2):135-168. PMID: 25915736.

    HD2: Plants possess a family of HDAC proteins, the HD2 family, which is not found in animals or fungi and is distantly related to cis-trans isomerases found in insects, S.cerevisiae and parasitic apicomplexans. Using maize HD2 as a query, four candidate proteins, HDT1, HDT2, HDT3 and HDT4, were identified in the Arabidopsis proteome. The conserved N-terminus of these proteins contains the HD2-type HDAC domain of approximately 100 amino acids. The proteins are comprised of a conserved N-terminal domain, a central acidic domain and variant C-terminal domain. (1)

Reference
1. Pandey R, Müller A, Napoli CA, Selinger DA, Pikaard CS, Richards EJ, Bender J, Mount DW, Jorgensen RA. Analysis of histone acetyltransferase and histone deacetylase families ofArabidopsis thalianasuggests functional diversification of chromatin modification among multicellular eukaryotes. Nucleic Acids Res.2002;30(23):5036-5055. PMID: 12466527.