(1)Selection of recombinants due to the inactivation of antibiotics is a cumbersome procedure because it requires simultaneous plating on two plates having different antibiotics.
(2)Therefore, altenative selectable markers have been developed which differentiate recombinants from non-recombinant DNA.In this method,a recombinant DNA is inserted within the coding sequence of an enzyme,$\alpha$-galactosidase.
(3)This results into activation of the enzyme, which is reffered to as insertional inactivation.
(4)The presence of a chromogenic substuate gives blue-coloured colonies if the plasmic in the bacteria does not have an insert.The presence of insert results into insertional on activation of the $\alpha$-galactosidase, and the colonies do not produce any colour and identified as recombinant colonies