A synthetic polynucleotide and a method are disclosed for selectively expressing a protein in a target cell or tissue of a mammal. Selective expression is effected by replacing at least one existing codon of a parent polynucleotide encoding a protein of interest with a synonymous codon to produce a synthetic polynucleotide having altered translational kinetics compared to the parent polynucleotide. The synonymous codon is selected such that it has a higher translational efficiency in the target cell or tissue relative to one or more other cells or tissues of the mammal.