Cosmid library construction

To study the organization and regulation of eucaryotic genes from cloned DNA, vectors capable of maintaining large inserts have been constructed, largely from bacteriophage λ, where nonessential intergenic regions were identified. It had been noted that only λ DNA 37-52 kb in length (or 78-102% of the wild-type length) were efficiently packaged and passaged in vivo. Thus vectors from which the intergenic region had been removed could be positively selected for by the insertion of target DNA, thus restoring a packagable DNA length and producing viable phage.