C646 is a potent, selective and cell-permeable p300/CBP histone acetyltransferase (HAT) inhibitor with an IC50 ~400 nM. It competes with acetyl-CoA for the p300 Lys-CoA binding pocket selectively. It blocks the growth of human melanoma, leukemia, lung, and prostate cancer cells in vitro. C646 has also been used to study the role of p300-mediated gene regulation in neuroscience. The histone acetyltransferase (HAT) p300/CBP is a transcriptional coactivator implicated in many gene regulatory pathways and protein acetylation events. C646 is a useful chemical probe to study p300/CBP HAT functions.