DNA relaxation is an essential process of DNA replication in the bacterial or mammalian cells. The primary function of topoisomerase I is DNA relaxation.  Bacterial topoisomerase IV and human topoisomerase II also have DNA relaxation activities.  The Topoisomerase DNA relaxation 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.