When expressing proteins for drug discovery studies, it is important that the expression system delivers the critical characteristics of the proteins being expressed, such as proper folding, glycosylation, etc. While bacteria and yeast-based expression systems can provide rapid and cost-effective protein production, they are unable to generate most of the critical post-translational modifications needed for drug discovery work.

Fortunately, in the recent years, significant advances have been made to mammalian transient expression systems, leading to easy protein expression protocols and highly competitive protein yields because the systems are easily scalable. Therefore, mammalian expression systems are predominately chosen for this task, withHEK293andCHOcells being the preferred option. Actually, 70% of therapeutic proteins are manufactured in CHO cells.