One member of particular interest in the family of acyltransferases is the group of histone acetyltransferases (HATs; EC 2.3.1.48). Enzymes of this kind acetylate core histones, which results in important regulatory effects on chromatin structure and assembly, and gene transcription. In the nucleus of eukaryotic cells,DNAis highly compacted and organized into chromatin, whose basic unit is the nucleosome, composed by DNA and an octamer of core histones (H2A, H2B, H3, H4). The histones expose their N-terminal tails out of the octamer. These tails can be highly post-translationally modified, leading to the transcription regulation. While histone acetylation is a dynamic reversible process, the balance of histone acetylation is important for proper cellular function. Based on their catalytic domains, HATs can be grouped into three groups, mainly: the GNATs (Gcn5 N-acetyltransferases), the 60 kDa Tat interactive protein (MYSTs) and the orphan HATs. P300/CBP-associated factor (PCAF), Elp3, Hat1, Hpa2 and Nut1 belong to the first group, with the founding member, GCN5. Morf, Ybp2, Sas2 and Tip60 represent the second group. Not containing a precise consensus HAT domain, the third group is called ‘orphan’, although these enzymes show an intrinsic HAT activity. p300/CBP, for example, belongs to this group together with Taf1 and several nuclear receptor (NR) co-activators. Besides HATs, the cell has evolved enzymes that catalyze the removal of acetyl groups from histone as well, termed histone deacetylases (HDACs, section Enzymes (EC 3.5.1.)).