What is the role of lactose in regulating lac operon expression?

The function of lactose in regulating lac operon expression is to cause transcription by binding to the repressor and thereby remove it from the operator. Lac operon in E. coli is expressed only when lactose is present and glucose is absent. Two operators turns the operon on and off in response to lactose and glucose concentration, these are the lac repressor and the catabolite activator protein.