Inhibition of histone deacetylase (HDAC) has been implicated to modulate transcription and induce apoptosis or differentiation in cancer cells. However, screening compounds for HDAC inhibition has been difficult due to the lack of convenient tools for analyzing HDAC activity. The Fluorometric HDAC Activity Assay Kit provides a fast and fluorescence-based method that eliminates radioactivity, extractions, or chromatography, as used in traditional assays. The new procedure requires only two easy steps, both performed on the same microtiter plate. First, the HDAC substrate, which comprises an acetylated lysine side chain, is incubated with a sample containing HDAC activity (e.g., HeLa nuclear extract). Deacetylation of the substrate sensitizes