A method of designing a polynucleotide sequence encoding a polypeptide sequence of a predetermined polypeptide is provided. A frequency lookup table corresponding to an expression system is obtained. The table comprises a plurality of sequence elements and a plurality of frequency ranges, each frequency range for a corresponding sequence element. Each frequency range is a range of frequencies with which a corresponding sequence element can occur in a polynucleotide. The polynucleotide sequence is defined using the frequency lookup table by determining, for each respective sequence element in the frequency lookup table, whether the respective sequence element encodes a portion of the polypeptide sequence. When the respective sequence element encodes a portion of the polypeptide sequence, the sequence element is incorporated into the polynucleotide at a frequency of occurrence that is within the frequency range specified for the respective sequence element in the lookup table. The polynucleotide sequence is then outputted.