First paragraph (this article has no abstract)

Isolation of mammalian cell clones for high-level protein production remains usually impeded by the time-consuming processes of selection, gene amplification and the analysis of many clones that is required to identify one with favorable properties, while maintaining proper protein properties and consistency. Expression variability results in part from the site of transgene integration in the host genome, and from the variable number of transgene copies that integrate. We and others have shown that genetic insulator elements such as MAR can be used to shield transgenes from inhibitory effects of the surrounding chromosomal sequences, alleviating in part integration site effects [1-4]. However, productivity remains limited by the number of transgenes that can be intergrated in the host genome. Thus, it would be useful to increase the number of integrated transgenes, and to render this integration process more frequent and more reproducible.