The lysogenic phase of infection is also used in cloning technology. Genes or foreign sequences may be incorporated essentially permanently into the genome of E. coli by integration of a vector containing the sequence of interest. One example of the use of this method is a strain used for overexpression of proteins by the T7 method. The strain BL21(DE3) and derivatives include the gene for T7 RNA polymerase under control of the lac promoter as a lysogen, designated DE3. The gene can be induced by IPTG, and the polymerase will then transcribe the target gene in the expression vector
