A method of making a genetically modified mammalian cell, 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 a first type of mammalian cell than the first codon in a comparison of translational efficiencies of codons in cells of the first type, replacing the first codon with the synonymous codon to form a synthetic polynucleotide, and introducing the synthetic polynucleotide into a mammalian cell to produce the genetically modified mammalian cell.