Why is it recommended to add 1 mM IPTG for optimal protein yields using the EasyXpress Protein Synthesis Kits?
FAQ ID -860

Slight variations in the endogenous level of Lac repressor when using plasmid DNA with an E. coli promoter in the EasyXpress Protein Synthesis System may result in slightly reduced transcription/translation efficiency. This can be overcome by addition of 1 mM IPTG. When T7 promoter-based plasmids are used, addition of IPTG is not required.