Butyrolactone 3 (MB-3) is a specifical small-molecule inhibitor of the histone acetyltransferase Gcn5 (IC50=100 μM), which has a high affinity to the Gcn5 enzyme comparable to that of its natural substrate, histone H3. Butyrolactone 3 shows weak inhibitory on CBP (IC50=0.5 mM). Butyrolactone 3 can be used in studies of cancer, metabolic, autoimmune and neurological diseases.