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.