Histone acetyltransferases (HAT) are enzymes that play a critical role in transcriptional regulation of genes, and directly correspond to transcription by selectively acetylating the epsilon-amino groups of lysines located near the amino termini of core histone proteins. Abnormal gene silencing by reduced HAT activity has been linked to the pathogenesis of many diseases, particularly cancer. Determining the activity of HATs and the potency of their inhibitors contributes to drug discovery and the development of anti-cancer agents, as well as a better understanding of gene transcription.