Used primarily in molecular biology for blue-white screening in recombinant cloning. When added to bacterial culture media, it acts as a chromogenic substrate for β-galactosidase. Cells expressing active β-galactosidase enzyme cleave X-gal, producing an insoluble blue pigment, allowing visual identification of colonies. In plasmid cloning, disruption of the β-galactosidase gene results in white colonies, indicating successful insertion of foreign DNA, while intact gene expression yields blue colonies. This enables rapid selection of recombinant clones without further testing. Also used in histochemistry to detect β-galactosidase activity in tissues, particularly in aging studies and gene expression analysis.