C646, a pyrazolone-containing small molecule, is a cell-permeable histone acetyltransferase (HAT) inhibitor that competitively and selectively inhibits the HAT activity of p300, a transcriptional co-activator involved in a variety of gene regulatory pathways and protein acetylation events, with the Ki value of 400 nM and an IC50 value of 1.6 µM .