FAQ: Why are there no colonies or no growth in liquid culture (C2530)?

This outcome may be due to basal expression of a target gene product, which is detrimental to cell viability. If target protein toxicity is suspected, strains that overproduce LacI (e.g. NEB #C3037 NEB Express Iq Competent E. coli) are recommended for vectors containing a tac, trc, lac, lacUV5 or T5-lacO promoter.