Complex recombinant protein biologics, such as monoclonal antibodies and coagulation factors, are key components of today’s biopharmaceutical industry. There are many effective paths for generating recombinant proteins in mammalian cells. Stable cell lines made in Chinese Hamster Ovary (CHO) cells are the workhorse for production of biopharmaceuticals due to their relative ease of use and long history of regulatory acceptance. However, in development and laboratory settings, where large numbers of proteins are generated for pre-clinical studies, transient gene expression using the FS293F™, Expi293F™ and ExpiCHO™ systems is an efficient, rapid and cost-effective alternative to developing stable CHO cell lines. Transient gene expression technology has allowed us to rapidly screen, identify and characterise multiple novel protein-based human therapeutic drug candidates. Notably, we have observed that the choice of expression system has a great influence on not only the quantity but also the quality of the produced recombinant protein. The Freestyle 293™, Expi293™ and ExpiCHO™ cell lines are excellent hosts for robust secretion of mammalian proteins, however the cellular machinery for appropriate post-translational modifications for particular proteins is not always optimal. We are currently developing tools that are expected to enable the generation of proteins with appropriate post-translational modifications and hence the desired biological activity and pharmacokinetics.