Choosing proper restriction enzymes based on defined criteria for PCR cloning. (A) Two single-cutter restriction enzymes (E1 and E2) are located downstream of the promoter. (B) E1 and E2 restriction enzymes cut the plasmid downstream of the promoter several (here two times for each enzyme) times. (C) The E1 restriction enzyme cuts the plasmid downstream of the promoter more than once. (D) The PCR product, which contains the tdTomato gene and the restriction enzyme sites, was run on a gel before being extracted for downstream applications.