Genetic control involves genes switching on and off to regulate cellular differentiation and metabolic pathways. Jacob and Monod proposed that in E. coli, the lac operon contains genes for digesting lactose that are switched on by the presence of lactose. The regulator gene produces a repressor that binds to the operator gene and switches off the structural genes unless lactose is present. Phenylketonuria is caused by a mutation preventing breakdown of the amino acid phenylalanine, causing intellectual disability if untreated by diet. Cell differentiation is controlled by certain genes switching on in specific cell types only.