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E.coli is one of the most commonly used protein expression systems and protein expression is usually induced using a DNA plasmid expression vector. This bacterium can be grown easily and inexpensively under standard laboratory conditions and expression using this system is relatively simple, fast, and inexpensive. If a target protein originates from a prokaryotic system, E.coli is an easy choice. If, however, the protein is of eukaryotic origin, the PTMs cannot be guaranteed in this system and another system may be preferable. In the case that PTMs are not important in the target protein, E.coli can still be a good choice for even eukaryotic protein expression.

Baculovirus/Insect expression system allows for expression of high quality glycosylated proteins that cannot be expressed using E.coli or yeast. Insect cells are a higher eukaryotic system than yeast and are therefore better able to carry out more complex PTMs. They also have superior machinery for folding mammalian proteins and therefore give the best chance of obtaining soluble protein when a protein of mammalian origin is expressed.

Mammalian cells are more complex than bacterial cells and have the most sophisticated protein production capabilities including mechanisms to assist protein folding into the proper conformation like chaperones and modifications (glycosylation, methylation, phosphorylation etc.). For a protein with mammalian origin, the activity can be related not only to the primary structure (sequence), but also to the conformation and modifications, while other systems, especially when distantly related, cannot accomplish all the modifications found naturally in mammalian cells. This is a reason why many researchers believe the activity of mammalian proteins is best when expressed in a mammalian cell. GenScript employs genetically modified CHO and HEK293 cells together with elaborately designed expression vectors to give high yield and quality protein production, which has been proven by hundreds of successful projects.