The structural genes of the lac operon are transcribed into a single mRNA, which is then translated into the three enzymes.

The lacZ gene encodes β-galactosidase, an enzyme that digests the disaccharide lactose into glucose and galactose.

The lacY gene codes for the enzyme permease, a transport protein which facilitates the diffusion of lactose into the cell.

The lacA gene codes for the enzyme transacetylase.