Editing Cloning vector (section)

==Screening: example of the blue/white screen==
{{main|Blue white screen}}
Many general purpose vectors such as [[pUC19]] usually include a system for detecting the presence of a cloned DNA fragment, based on the loss of an easily scored phenotype. The most widely used is the gene coding for ''E. coli'' [[beta-galactosidase|β-galactosidase]], whose activity can easily be detected by the ability of the enzyme it encodes to hydrolyze the soluble, colourless substrate [[X-gal]] (5-bromo-4-chloro-3-indolyl-beta-d-galactoside) into an insoluble, blue product (5,5'-dibromo-4,4'-dichloro indigo). Cloning a fragment of DNA within the vector-based ''lacZα'' sequence of the β-galactosidase prevents the production of an active enzyme. If X-gal is included in the selective agar plates, transformant colonies are generally blue in the case of a vector with no inserted DNA and white in the case of a vector containing a fragment of cloned DNA.