Publisher Summary Plasmids are widely used as vectors and methods of their isolation can vary depending upon the host organism. On the basis of phenotypic effect and organization in the chromosomes, genes can be subdivided into various groups, such as simple genes, complex genes, operons, regulons, and multiple regulons. Any DNA segments obtained by shear—by using two different restriction enzymes or by using restriction enzymes that produce blunt ends—can be joined without adding homopolymer tails. However, blunt-end ligation does not allow the recovery of the cloned fragment after the recombinant DNA has been propagated. The restriction site DNA linkers used for manipulating DNAs that lack cohesive ends (sheared DNA, synthetic DNA, cDNA)—are commercially available. If the DNA fragment is not flush-ended, it can be first digested with the Aspergillus nuclease S 1 to produce even ends as this linker can be used with several restriction enzymes. Individual competent cells can be transformed by more than one plasmid simultaneously, which allows for the indirect selection of a cryptic plasmid if the transformation is carried out with a mixture of a cryptic plasmid and a small selectable plasmid.