How do you select an appropriate expression system for recombinant protein production?

Expression system selection depends on protein requirements. E. coli offers rapid, inexpensive production but lacks post-translational modifications (PTMs) and may form inclusion bodies with complex proteins. Yeast (Pichia, S. cerevisiae) provides eukaryotic PTMs and secretion at moderate cost. Insect cells (baculovirus system) produce properly folded proteins with most PTMs except human-type glycosylation. Mammalian cells (CHO, HEK293) provide authentic human PTMs essential for therapeutic proteins but are expensive and slow. Plant and transgenic animal systems suit specific applications. Consider also scale requirements, downstream processing, and regulatory pathways.