Chromogenic substrate of β-Galactosidase used in combination with IPTG for detection of β-Galactosidase activity in bacterial colonies. X-GAL is cleaved at the B1-4 bond between galactose and the 5-Bromo-4-chloro-3-indolyl portion of X-GAL by β-Galactosidase via hydrolysis. The cleavage of X-GAL results in the production of a water insoluble blue dichloro-dibromo-indigo precipitate. In cloning strategies with vectors such as Lambda-11, M13mp18 and 19, pUC18 and 19, pUR222 the E. coli lacZ gene is transformed to lac+ cells. After transformation the cells show β-Galactosidase activity in the presence of media containing X-GAL and IPTG. The insertion of a DNA fragment into the cloning sites of the lacZ gene results in the disruption of β-Galactosidase activity leading to the appearance of white colonies. Non-recombinant cells produce a blue indigo dye.
