Blue/white Selection

A technique usually used to identify recombinant colonies on agar for use in PCR.

E.coli cells are often used as hosts in this technique, containing the β-peptide in the genome. The α-peptide of β-galactosidase is expressed in the plasmid, when DNA is inserted into the lacZα gene it disrupts the formation of β-galactosidase. β-galactosidase is needed to metabolise X-gal into a bright blue product, therefore the recombinant plasmid containing the DNA insert will not be able to form the blue product and so all colonies with recombinant plasmid would be white.