What expression systems are used for producing therapeutic proteins?

Different expression systems are chosen based on protein requirements: 1) E. coli - fast, inexpensive, high yields; lacks glycosylation; used for insulin, growth hormone. 2) Yeast (S. cerevisiae, Pichia pastoris) - eukaryotic folding, simple glycosylation; hepatitis B vaccine, insulin. 3) Mammalian cells (CHO, HEK293) - complex human-like glycosylation essential for antibodies; most therapeutic proteins; expensive, slower. 4) Insect cells (baculovirus system) - post-translational modifications, good for complex proteins; vaccines. 5) Plant cells - scalable, no animal pathogens; some vaccines and enzymes. 6) Transgenic animals - for rare proteins; antithrombin from goat milk. Selection factors include glycosylation needs, protein complexity, cost, scalability, and regulatory considerations.