The lac repressor (LacI) is a DNA-binding protein that inhibits the expression of genes coding for proteins involved in the metabolism of lactose in bacteria.  These genes are repressed when lactose is not available to the cell, ensuring that the bacterium only invests energy in the production of machinery necessary for uptake and utilization of lactose when lactose is present.  When lactose becomes available, it is firstly converted into allolactose by β-Galactosidase (lacZ) in bacteria. The DNA binding ability of lac repressor bound with allolactose is inhibited due to allosteric regulation, thereby genes coding for proteins involved in lactose uptake and utilization can be expressed.