Lac operon is the operon that regulates lactose metabolism in the bacterium Escherichia coli. Its form was first postulated in 1961 by Francois Jacob and Jacques Monod to explain control of $\beta$-galactosidase synthesis and for this work they were awarded Nobel Prize. Lac operon system or inducible operon system consists of regulator gene (i), promotor gene (p), operator gene (o), three structural genes ( $\mathrm{z}, \mathrm{y}$ and $\mathrm{a})$, repressor protein and inducer.