A: The basic principle of primer design is the target gene can be amplified specifically. Usually the length of the primer is 24-27 nucleotides. The annealing temperature is about 60 degrees. The GC content of target fragment should not less than 48%; the length of target fragment should not less than 800bp. In case there is a particular gene structure, above parameters can be adjusted, but the quantity of mutants can not be guaranteed.