5-Bromo-4-Chloro-3-Indolyl--D-Galactopyranoside is used as indigogenic substrate for -galactosidase, for detection of -galactosidase-positive clones, and the identification of lac and bacterial colonies or phage plaques. It is the substrate of choice for blue-white selection of recombinant bacterial colonies with the lac+ genotype. XGal is cleaved by galactosidase to yield an insoluble blue precipitate. XGal is particularly useful in molecular biology applications to detect the activity of galactosidase which is frequently used as a reporter gene. In cloning, XGal is used to detect insertion of foreign DNA into the lacZ region of plasmid DNA using -complementation which is based on vectors such as the pUC and the M13mp series that carry a fragment of the -galactosidase gene encoding an -fragment of -galactosidase.