Blue/white Selection

Blue/White selection is a technique that can be used to identify recombinant colonies on agar for use in PCR.

Escherichia 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, preventing the plasmid from being able to transcribe and therefore express β-galactosidase.

When β-galactosidase is expressed, the substrate X-gal can be metabolised, producing a blue colony. However, the recombinant plasmid that contains the DNA insert will not be able to produce blue colonies and instead will produce white colonies.