Early in the development of DNA vaccines, it became clear that maximizing the expression of the encoded Ag was critical to the induction of potent immune responses. Strong viral promoters, such as CMV-intA, are generally favoured over regulated or endogenous eukaryotic promoters [70] . Furthermore nuclear targeting sequence (NTS) could be introduced to increase the efficiency of nuclear plasmid uptake from cytoplasm after intramuscular injection [48, 71] . The utilization of codon-optimized sequences instead of the wild-type coding sequences is a general and potent method to improve vaccination. An optimal coding sequence is determined back from the amino acid sequence of the antigen by algorithms that take into account the abundance of specific tRNAs in the cytosol of human cells and the predicted structure of the mRNA. Thereafter the selected gene sequence is constructed in vitro using synthetic oligonucleotides. Adverse rare codons are avoided and secondary structures in the mRNA are minimized. Thereby, the synthetic gene is optimal for expression and consequently for the induction of an immune response [72] .