The Act5C promoter is fused upstream of sequence encoding an N-terminal myristoylation signal (Tag:Myr(Src64B)) and an EGFP coding sequence in which the reading frame has been interrupted by inserting a CaSSA switch cassette that contains target sites for both the 'gRNA#1' and 'gRNA#2' guide RNAs, with the target sites flanked on each side by a 400bp direct repeat of part of the EGFP sequence. Designed as part of the CaSSA system: if a double-strand break is induced using Cas9 and a source of either the 'gRNA#1' or 'gRNA#2' guide RNA, strand annealing (SSA) repair can reconstitute a functional EGFP reporter that is expressed in the cell in which the repair occurs and also in any of its progeny.