nos regulatory sequences (promoter and 3'UTR) regulate expression of Cas9, which has been tagged with three copies of the ligand-regulated Tag:degron(ecDHFR) destabilizing domain; one at the N-terminal end, one in an loop and the third at the C-terminal end. In the absence of trimethoprim (TMP), the expressed protein is rapidly degraded by the proteasome (due to the presence of Tag:degron(ecDHFR)). In the presence of TMP, only low Cas9 editing activity is seen.