HDACs catalyze the hydrolytic removal of acetyl groups from histone lysine residues resulting in chromatin condensation and transcriptional repression of chromosomal DNA. Thus, HDAC inhibition allows the conformation of DNA to be relaxed and transcriptional activation to ensue. Transcriptional repression of key genes has linked HDAC activity to cell growth and tumor development. Cayman's HDAC Activity Assay provides a fast, fluorescence-based method for measuring Class I and II HDAC activity that eliminates radioactivity, extraction, or chromatography. The procedure requires only two easy steps, both performed in the same microplate. The fluorescent reaction product is analyzed using a plate reader with excitation wavelengths between 340-360 nm and emission wavelengths between 440-465 nm.