X-Gal (CAS 7240-90-6) is a chromogenic substrate specifically hydrolyzed by β-galactosidase, resulting in a blue-colored insoluble product, 5,5'-dibromo-4,4'-dichloro-indigo. Structurally, X-Gal is a galactopyranoside derivative cleaved enzymatically into galactose and a colored indigo dye. In molecular biology, it serves predominantly in recombinant DNA technologies to differentiate recombinant clones via blue-white colony screening. When bacteria host plasmids containing the lacZα fragment of β-galactosidase gene, complementation with host ω fragment yields functional enzyme hydrolyzing X-Gal, producing blue colonies. Recombinant plasmids disrupted by inserted exogenous DNA cannot complement enzymatic activity, generating white colonies. Thus, X-Gal testing facilitates rapid visual identification of successful recombinant inserts.