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E.coli strains encoding the T7 RNA polymerase gene are especially useful for robust over-expression of protein.For many years,NEB scientists have relied on genetically-tailored T7 expression strains for the over-production of enzymes.The NEB "T7 Express" strain is a BL21 derivative with several unique features.First,the T7 RNA polymerase gene is encoded within the lac operon and is expressed from the wt lac promoter.This results in a lower basal expression of the target protein as compared to strains carrying the lambda DE3 prophage where T7 RNA polymerase expression is under lac UV5 control.Furthermore,T7 Express strains carry the sulAll mutation,which enables E.coli B strains to better recover from an SOS response (1).