One reason for these higher yields is that the GPEx® retrovector targets high-expressing chromosomal matrix attachment sites in the target cell genome, leading to markedly higher expression of the gene in transduced cells compared with other systems. Another important reason is that the optimization of GPEx® cell lines employs an iterative insertion process that drives up the inserted gene copy number and proportionally increases protein expression levels. The optimized cell line may therefore have a dozen or more copies of the desired gene, all stably inserted and expressing the target protein.