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The optimization of single factors in a CHO-based transient expression system provides modest gains, at best. In many instances existing transient CHO systems express at 10-50mg/L titers, such that even a 50% increase in titer is not a significant improvement. The ExpiCHO system has been developed from the ground up by identification of a high expressing CHO clone, ExpiCHO-S cells, a new expression media and feed, as well as a new high-efficiency transfection reagent and expression enhancers, all of which when optimized to work together increased protein titers of a human IgG from 20mg/L in the FreeStyleCHO transient system to 3000mg/L in the ExpiCHO system. It is truly through the synergistic interaction of all of the components of the system where these log-fold improvements in protein titers can be observed.