Mammalian cell expression systems are the best choice for the production of eukaryotic proteins, especially when correct folding and post-translational modification (glycosylation, phosphorylation, etc.) is required. They produce eukaryotic recombinant proteins in the most natural state, with native tertiary structure, physiochemical characteristics, and bioactivities. They have been successfully applied in the biopharmaceutical production of cytokines, monoclonal antibodies, growth factors and so on. The most widely-used mammalian cell lines are HEK293 and CHO cells.