(a) In repression, an active repressor (encoded by the R gene in this example) blocks expression of the A, B, C operon by binding to an operator site (O). (b) In activation, a functional activator is required for gene expression. A nonfunctional activator results in no expression of genes X, Y, Z. Small molecules can convert a nonfunctional activator into a functional one that then binds to the control region of the operon, termed I in this case. The positions of both O and I with respect to the promoter P in the two examples are arbitrarily drawn, inasmuch as their positions differ in different operons.