BL21(DE3)pLysS allow protein expression of any gene that is under the control of a T7 promoter. BL21(DE3)pLysS is lysogenic for λ-DE3, which contains the T7 bacteriophage gene I, encoding T7 RNA polymerase under the control of the lac UV5 promoter.
The pLysS plasmid carries the gene encoding T7 lysozyme, which lowers the background expression level of target genes under the control of the T7 promoter without interfering with the level of expression achieved after IPTG induction.