BL21(DE3) E. coli are ideal for use with bacteriophage T7 promoter-based expression systems (e.g., pRSET, pCR¤T7, and pET). BL21(DE3) carry the lambda DE3 lysogen. Recombinant proteins that are non-toxic to E. coli are generally expressed at higher levels in BL21(DE3) cells than in BL21(DE3)pLysS or BL21(DE3)pLysE. However, the basal expression levels of heterologous genes are significantly higher in
BL21(DE3) than in BL21(DE3)pLysS or BL21(DE3)pLysE.