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Blue‐white screening is also based on insertional inactivation of a gene giving blue color compound as a visual representation of the result. The gene that plays a key role in this screening is lacZ gene which codes for the enzyme β‐galactosidase is under the control of the inducible lac promoter. lacZ gene is repressed by lac repressor and is induced by IPTG (isopropyl‐β‐D‐thiogalactopyranoside). The enzyme formed can breakdown X‐gal (five‐bromo‐4‐chlore‐3‐indolyl‐β‐D‐galactopyranoside) to give a blue color compound.

Insertional inactivation of lacZ gene due to insertion of desired gene prevents the formation of the enzyme, breakdown of X‐gal, and hence no blue color is obtained. The cells are plated on agar plates having IPTG, X‐gal, and an antibiotic giving combination of blue and white colonies. The transformed cells with religated vector having functional lacZ gene shows blue color colonies. Whereas the cells with gene of interest and disrupted lacZ gene forms colorless colonies. Further the colorless colonies can be picked and plated again.