Expression of heterologous proteins presents many challenges. In E. coli, expression of a non-native protein often adversely affects the viability of the host cell both during the transformation stage and during protein expression. To improve host viability and consequently improve the potential for target protein over-expression, well-regulated expression systems should be employed. In E. coli expression systems, regulation is often provided by the host cell as well as the expression vector. IPTG-inducible systems rely on the Lac repressor to bind to lac, tac, trc or T7-lac promoters to inhibit expression until the culture reaches an optimal density for induction. In T7 systems, the Lac repressor is also important but an even more effective means to control expression is to employ a host strain that expresses a T7 RNA polymerase inhibitor protein (e.g. LysY).