In order to maximize protein expression, the Staby® system was combined to the proven T7 polymerase technology for the over-expression of recombinant proteins: the gene of interest is under the control of the T7 promoter. This combination is called StabyExpress®. The pStaby1 plasmid contains the gene encoding the antidote protein (ccdA) which counteracts the action of the ccdB protein. The SE1 strain, in which the ccdB selection gene is inserted in the chromosome in order to stabilize the pStaby1 plasmid has been engineered by Delphi Genetics as a dedicated bacterial strain for protein expression and is derived from BL21(DE3). Consequently, IPTG has to be used to induce protein expression, an alternative strategy being the use of an auto-inducible medium (such as Delphi Genetics' Staby®Switch medium). Staby®Express ensures that only plasmid-bearing clones can grow whereas every clone losing the plasmid will inevitably die.