β-Galactosidase gene codon optimization results in post-transcriptional enhancement of expression.

•lacZ codons optimized with 7 highly abundant cow milk proteins to construct olacZ.•olacZ expression increased in vitro, in cell lines 293T, HeLa, Cos7, NIH 3T3, HC11.•olacZ expression increased in vivo, in breast tissue in transgenic lactating female mice.•olacZ expression enhancement observed at the translational level.•System-preferential codon optimization can be used with other organs or bioreactors.