The properties of this strain that contribute to its usefulness as a protein expression strain are described below. The genotypes underlying these properties appear in parentheses. T7 RNA Polymerase: (T7 gene1) is encoded by the lambda DE3 prophage present within the chromosome. T7 RNA polymerase is expressed from the lacUV5 promoter, which is less sensitive to catabolite repression than the wt lac promoter. Thus DE3 strains may exhibit uninduced target protein expression. Although λDE3 is normally dormant in the host chromosome, the induction of the SOS cascade can occur as the result of expressing proteins that damage the E. coli chromosome, either directly or indirectly. This may lead to cell lysis. T7 Express strains do not carry the DE3 prophage and better tolerate an SOS response. Protease Deficient ([lon] ompT): E. coli B strains are “naturally” deficient in the lon protease which in K-12 strains serves to degrade misfolded proteins and to prevent some cell cycle-specific proteins from accumulating. The OmpT protease resides at the surface of wild type E. coli in both K-12 and B strains, presumably helping the cells to derive amino acids from their external environment. Cells deficient in both these proteases are much more amenable to the production of proteins from cloned genes. T1 Phage Resistant (fhuA2): T1, an extremely virulent phage requires the E. coli ferric hydroxamate uptake receptor for infectivity. Deletion of this gene confers resistance to this type of phage, but does not significantly affect the transformation or growth characteristics of the cell.