The Lac operon is regulated by two specific proteins, being one of them responsible to prevent the RNA polymerase from transcribing, and the other that the RNA polymerase bind to the promoter. In the internal environment the the lac operon uses lactose from a food source to starts working, while if there is not lactose the lac operon is switched off by block proteins. Therefore, it can be regulated by the presence of lactose and the lac regressor gene.