The DNA Topoisomerase I Assay Kit is based on the principle that the supercoiled DNA and relaxed DNA yield different fluorescent intensity when interact with fluorescence dye H19. The relaxed DNA suppresses the fluorescent intensity much more than the supercoiled DNA. Therefore, when the supercoiled DNA is converted into its relaxed form, the fluorescent signal decreases. The change of fluorescence intensity is used to measure the relaxation reaction. Each assay kit includes the reaction buffer (T1 Buffer), supercoiled plasmid DNA and fluorescence dye H19 for assaying 20 DNA relaxation reactions in a 96-well plate assay format or 40 assays in a 384-well assay format. The reaction buffer is optimized for bacterial topoisomerase I.