ZFN’s can only change DNA sequences they are designed for. The DNA-binding domain binds to a specific DNA sequence. Two ZFN’s designed for the same DNA sequence are needed change one gene. The DNA-binding domains bind to both strands on the DNA double helix. DNA-cleaving domains break the DNA strand and cut the gene base pairs out. Cells fix broken DNA strands on their own using DNA repair. Deletion mutations result from missing base pairs after DNA repair. 