In the second case, expression is induced by the addition of IPTG to the
bacterial culture. Although in some cases (e.g., with innocuous target proteins) it may be
possible to clone directly into expression hosts, this approach is not recommended as a general
strategy. Two types of T7 promoter and several hosts that differ in their stringency of
suppressing basal expression levels are available, providing great flexibility and the ability to
optimize the expression of a wide variety of target genes.