The increased restriction sites in the polylinker region increase the opportunity for the target DNA fragment to be inserted into the plasmid, and if embedded in the lacZ gene, it increases the liklihood that the gene will be interrupted by the insertion of the DNA. If the lacZ gene is interrupted, B galactosidase is not produced. When plated on X-gal medium, the resulting colonies containing the recombinant DNA will turn white from the inability to cleave X-gal with B galactosidase, as opposed to the blue colonies with a functional, uninterrupted lacZ gene. If the polylinker region is not embedded in the lacZ component, the liklihood that the recombinant DNA will disrupt the lacZ gene is not as high. The advantage creates an increased liklihood of a selective marker that determines the colonies that have recombinant DNA from those with nonrecombinant DNA, based on their color.