In molecular biology, an inducer is a molecule that regulates gene expression. An inducer functions in two ways; namely:

• By disabling repressors. The gene is expressed because an inducer binds to the repressor. The binding of the inducer to the repressor prevents the repressor from binding to the operator. RNA polymerase can then begin to transcribe operon genes.
• By binding to activators. Activators generally bind poorly to activator DNA sequences unless an inducer is present. Activator binds to an inducer and the complex binds to the activation sequence and activates target gene. Removing the inducer stops transcription.

Because a small inducer molecule is required, the increased expression of the target gene is called induction. The lactose operon is one example of an inducible system.