The invention relates to a method of indirectly identifying a polynucleotide comprising an open reading frame having an improved expression level from a library of polynucleotides encoding the same polypeptide of interest, or variants or homologues thereof, in a host cell expression library, the method comprising the steps of: a) providing an expression library in a host cell, said library comprising at least two different open reading frames encoding the same polypeptide of interest, or variants or homologues thereof, each open reading frame being translationally coupled to a downstream gene encoding a screenable or selectable reporter; b) culturing the host cell under conditions conducive to the expression of the polypeptide of interest, or variant or homologue thereof; and c) screening and/or selecting for a host cell having an improved expression level of the translationally coupled reporter when compared to one or more other host cells of the library, thus indirectly identifying a polynucleotide comprising an open reading frame having an improved expression level in the host cell; as well as the expression libraries and host cells comprising said libraries.