T7 expression in BL21(DE3) is not tightly regulated and thus toxic proteins may affect cell viability, especially if the expression vector does not encode lacI. For tightly regulated expression, use a strain expressing lysY: T7 Express lysY (NEB #C3010): lysY produces mutant T7 lysozyme which binds to T7 RNA polymerase, reducing basal expression of the target protein. Upon induction, newly made T7 RNA polymerase titrates out the lysozyme and results in expression of the target protein T7 Express lysY/Iq (NEB #C3013): lysY expression as well as lacI over-expression to repress basal expression of the T7 RNA Polymerase. Lemo21(DE3) (NEB #C2528): BL21(DE3) containing the Lemo System™. lysY expression is modulated by L-rhamnose, making T7 protein expression tightly regulated and tunable.