When a gene of interest is inserted within the coding sequence of -galactosidase enzyme ( select- able marker),if the receipient cell fails to develop blue colour in presence of a chromogenic substrate, it is an indicative of successful recombinant formation and presence of blue colour show no recombinants formed , whereas when an antibiotic resistant selectable markers are used it requires simultaneous plating of two different plates having different antibiotics, to differentiate between transformants from non - transformants and then select the transformants to identify the recombinants from them , which is a very time consuming and cumbersome process in comparision to the first one
