How do you optimize transient expression in mammalian cells for protein production?

Transient expression optimization involves: cell line selection (HEK293, CHO, Expi293 for high-density suspension culture), transfection optimization (PEI:DNA ratio, timing, cell density at transfection), plasmid design (strong promoters like CMV/CAG, optimized Kozak, introns, codon optimization), culture conditions (temperature reduction post-transfection to 32-34C, feed supplements, valproic acid for histone deacetylase inhibition), and harvest timing (typically 5-7 days). For multi-subunit proteins, optimize plasmid ratios. Process intensification includes perfusion during expression phase. Analytics include measuring viable cell density, product titer, and quality attributes. Scale considerations differ from stable expression - transient is faster but less consistent. Applications include rapid protein production for research, antibody generation, and early-stage material for developability assessment.