When a reporter enzyme such as the Lac Z gene is present in the genome of a transformed cell it produces the enzyme β–galactosidase. The substrate on which β–galactosidase acts on is lactose. When lactose is present in the culture medium as the carbon source of the bacteria β – galactosidase breaks down lactose to produce the end product X – gal which gives a characteristic blue colour. This blue colour is visible with naked eye and by observing the culture plates it can be identified which of the host cells are transformed. When a gene of interest is incorporated within this Lac Z gene the gene becomes inactivated as it gets disrupted. This is known as Insertional inactivation. When such a plasmid transforms the host bacterial cell it does not develop any colour and thus can be identified. This process of identification is known as the blue–white screening process.