An expression system is the combination of a host organism or cell line and the molecular tools — such as vectors, promoters, and regulatory elements — used to produce a recombinant protein or biological molecule of interest. Common expression systems used in biopharmaceutical manufacturing include mammalian (e.g., CHO, HEK293), microbial (e.g., E. coli, Pichia pastoris), insect (e.g., Sf9), and plant-based systems, each offering different advantages in terms of yield, post-translational modifications, scalability, and regulatory acceptability.
