It is important to first note that activators and repressors can achieve exactly the same regulatory goals. For example, a gene that is fully expressed only in the presence of a signal (Figure 11.1), can be regulated by one of two mechanisms: Either an activator binds the promoter to activate the gene, or a repressor falls oἀ the promoter to activate the gene. These two mechanisms realize the same input-output relationship: Expression is turned on by the binding of an activator in the positive mode of control, and by the unbinding of a repressor in the negative mode of control. More generally, a gene controlled by N regulators, each of which can be either an activator or a repressor, has 2N possible mechanisms that can generate a given input-output mapping.