The present invention provides methods of reducing the levels of a titratable selectable pressure required, the number of amplification cycles, and the time taken to generate protein expressing cell lines by altering the codons of the desired open-reading-frames. Through the use of codon adaptation for this purpose the methods of the invention consistently provide sufficient yields in faster time frames saving many weeks in cell line development activities. Furthermore the methods of the invention also generate cell lines with lower concentrations of selection and amplification agent than previously achievable. Accordingly lower levels of selection and amplification marker in the final cells lines are observed.
