A method of determining a property that affects expression of polynucleotides in an expression system. A plurality of polynucleotides, each encoding a polypeptide sequence, is constructed. An amino acid is encoded a plurality of times in both a first and second polynucleotide in the plurality. The amino acid is encodable by a plurality of synonymous codons including a first codon. The first codon is in the first polynucleotide at a first frequency relative to other synonymous codons, and is in the second polynucleotide at a second frequency relative to other synonymous codons. The first and second frequencies are different. Each polynucleotide is individually expressed in the expression system to measure an expression property value of the polynucleotides, thereby determining a property that affects expression of polynucleotides. The property is an effect that a frequency of use of one or more codons has on expression of polynucleotides in the expression system.