Transgene designed as a reporter of DNA double-strand break (DSB) repair by single-strand annealing. It contains a ubiquitin promoter upstream of two copies of GFP separated by a RS(I-SceI) site. A stop codon is present between the first copy of GFP and the RS(I-SceI) site, thus in the original construct only the first GFP coding sequence is translated, while the second GFP sequence forms part of the 3'UTR. Little or no GFP protein is expressed, probably due to the transcript being targeted by nonsense mediated decay due to the long 3'UTR. If a DSB is induced at the RS(I-SceI) site by the I-SceI endonuclease, the break can be repaired by non-homologous end-joining (NHEJ) or by single-strand annealing (SSA). Repair by NHEJ does not result in an increase in GFP protein. Repair by SSA removes one copy of the GFP coding sequence, shortening the 3'UTR and resulting in a substantial increase in the production of GFP protein.