The present invention provides an improved method for achieving efficient transcription and translation of modified transgene constructs in vector systems. The vector may be a lentiviral vector. Such a method facilitates the production of viral vector genomes with intact functional transgene sequences allowing stable integration of a transgene-containing viral vector genome into the germline of an animal such as a transgenic avian. The subsequent expression of the transgene results in a recombinant protein product being produced, which, in the case of a transgenic avian can result in the targeted production of the protein into the egg of the transgenic bird.