Construct contains two transcription units. In one, the snRNA:U6:96Ac ( U6:3 ) promoter drives expression of two gRNA sequences. In the other, lexAop regulatory sequences drive expression of a GFP single-strand annealing reporter (GSR). The reporter is composed of 1. EGFP coding sequence that is interrupted by a fragment that is targeted by the two gRNAs produced by the first transcription unit, 2. a 2A peptide and 3. nuclear GFP. In the absence of SSA, neither fluorescent protein is expressed. In the presence of a source of Cas9 activity, the two gRNAs can trigger double-strand breaks at the site of interruption in the EGFP coding sequence. Single strand annealing at the breaks results in reconstitution of the reading frame and expression of both a cytosolic EGFP and a nuclear GFP.