Tip60, MOZ, MORF, and HBO1 are found in humans. Tip60 has roles in the regulation of gene transcription, HBO has been found to impact the DNA replication process, MORF is able to acetylate free histones (especially H3 and H4) as well as nucleosomal histones. p300/CBP family Adenoviral E1A-associated protein of 300kDa (p300) and the CREB-binding protein (CBP) make up the next family of HATs. This family of HATs contain HAT domains that are approximately 500 residues long and contain bromodomains as well as three cysteine-histidine rich domains that help with protein interactions. These HATs are known to acetylate all of the histone subunits in the nucleosome. They also have the ability to acetylate and mediate non-histone proteins involved in transcription and are also involved in the cell-cycle, differentiation and apoptosis. Other HATs There are other proteins that have acetylating abilities but differ in structure to the previously mentioned families. One HAT is called steroid receptor coactivator 1 (SRC1), which has a HAT domain located at the C-terminus end of the protein along with a basic helix-loop-helix and PAS A and PAS B domains with a LXXLL receptor interacting motif in the middle. Another is ATF-2 which contains a transcriptional activation (ACT) domain and a basic zipper DNA-binding (bZip) domain with a HAT domain in-between. The last is TAFII250 which has a Kinase domain at the N-terminus region, two bromodomains located at the C-terminus region and a HAT domain located in-between. Histone deacetylase (HDACs) There are a total of four classes that categorize Histone Deacetylases (HDACs). Class I includes HDACs 1, 2, 3, and 8. Class II is divided into two subgroups, Class IIA and Class IIB. Class IIA includes HDACs 4, 5, 7, and 9 while Class IIB includes HDACs 6 and 10. Class III contains the Sirtuins and Class IV contains only HDAC11. Classes of HDAC proteins are divided and grouped together based on the comparison to the sequence homologies of Rpd3, Hos1 and Hos2 for Class I HDACs, HDA1 and Hos3 for the Class II HDACs and the sirtuins for Class III HDACs. Class I HDACs HDAC1 & HDAC2 HDAC1 & HDAC2 are in the first class of HDACs are most closely related to one another. By analyzing the overall sequences of both HDACs, their similarity was found to be approximately 82% homologous. These enzymes have been found to be inactive when isolated which led to the conclusion