Histone acetyltransferases (HATs) enzymes regulate the acetylation of histones and non-histone proteins. The acetylation of the e-amino groups of lysine residues present at histone tails correlates largely with transcriptional activation, but it is also involved in DNA replication, DNA repair and protein–protein interactions. HATs play major roles in the control of cell fate and their misregulation is implicated in the development of some human tumors. HAT p300 is a transcriptional coactivator that acetylates core histones facilitating chromatin decondensation and recruiting basic RNA polymerase machinery. The SensoLyte® HAT (p300) Assay Kit provides a convenient protocol for screening of enzyme inhibitors and for continuous assay of p300 activity. For added convenience, this kit includes two peptide substrates: histone H3 (1-21) peptide and non-histone p53 peptide (368-386). After incubation with acetyl CoA and the substrate, the p300 enzyme generates acetylated H3 or p53 peptide and CoASH. The thiol groups of CoASH can be detected with fluorogenic reagent at excitation/ emission=389nm/513nm.