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In addition to the high titers and the ability of the ExpiCHO system to express proteins that don’t express well, or at all, in 293 cells, the key reason to use CHO cells for transient protein expression is to reduce the risk of critical quality attributes of your protein changing when transitioning from early discovery into pre-clinical and finally clinical trials. One of the main goals during the development of the ExpiCHO transient expression system was to ensure a more seamless transition from early drug discovery through clinical production allowing for researchers to start their work in CHO and stay in CHO throughout the entire drug development process. To this end, it is well documented that post-translational modifications in 293 cells and CHO cells are distinct from one another, with glycosylation patterns and sialic acid content being two instances of differences between the cells types. The ExpiCHO expression system demonstrated glycosylation patterns for a human IgG that were highly comparable to those of the same antibody expressed stably in CHO-S cells; in contrast, the glycan patterns for the same antibody expressed in 293 cells were strikingly different, with significant differences observed in the percentages of G0F, G1F and G2F glycoforms compared to CHO-derived material. The maintenance of protein critical quality attributes is critical in ensuring similar bioactivity and pharmacokinetic profiles of therapeutic proteins from pre-clinical to clinical trials.