A method of constructing a synthetic polynucleotide, the method including selecting a first codon of a parent polynucleotide that encodes a polypeptide for replacement with a synonymous codon, wherein the synonymous codon is selected on the basis that it exhibits a higher translational efficiency in an epithelial cell of a mammal than the first codon in a comparison of translational efficiencies of codons in test cells of the same type as the epithelial cell; and replacing the first codon with the synonymous codon to construct the synthetic polynucleotide.