Histone deacetylase (HDAC) enzymes act as transcriptional repressors of genes through the deacetylation of lysine residues on histone proteins. Sirtuins (SIRTs) are a unique class of deacetylases (class III HDACs) that require NAD as a cofactor. Sirtuin 1 (SIRT1) is involved in regulation of tumor suppressor p53 and inhibition of apoptosis. It also represents a target for treatment of age-related diseases and type II diabetes. The SensoLyte® 520 FRET SIRT1 Assay Kit provides a convenient, two-step homogeneous procedure for measuring sirtuin 1 activity using FRET substrate. In the first step, an acetylated substrate is incubated with SIRT1 containing samples. Deacetylation of substrate first sensitizes it, so that, in the second step mixing with the SIRT developer generates a fluorophore, which can then be detected at Ex/Em= 490/520 nm. This kit can be used for high throughput screening of SIRT1 inhibitors/activators. The long wavelength fluorescence of FRET substrate is less interfered by the autofluorescence of cell components and test compounds. The nuclear extract HDAC (containing HDACs 1 and 2) does not deacetylate FRET sirtuin substrate. Assays are performed in a standard 96-well format.