In our laboratory we study the mechanisms of DNA repair in the test tube by mixing DNA that contains an error with purified DNA repair proteins. We can verify whether repair has properly occurred only after the reaction is completed. Here we propose to construct a fluorescent sensor that can report the kinetics of the repair reaction in real time. This sensor will be based on the well-characterized CRISPR-Cas9 nuclease. The sensor will be programmed to exclusively bind to repaired DNA, resulting in an increased fluorescent signal that can be constantly monitored.