Lac operon is the operon of E. coli associated with lactose metabolism. It is an inducible operon that consists of a regulator, a promoter, an operator and three structural genes z, y and a. The structural genes are activated in presence of lactose that acts as an inducer. The structural genes are normally inactivated, as a repressor molecule binds to an operator gene preventing transcription. These genes get activated when lactose binds to the repressor disabling it from binding with operator. Now operator gene becomes free, permitting transcription thus expression of structural genes.

Its expression can be explained with the schematic diagram of lac operon in open state as follows