X-Gal (5-Bromo-4-chloro-3-indolyl-ß-D-galactopyranoside) produces a dimethylformamide (DMF)-soluble, stable blue precipitate at the reaction site. X-Gal is the classical substrate used with ß-galactosidase in current methods of molecular biology, microbiology and immunology. X-Gal is used to detect vectors, plasmids or DNA fragments that contain the ß-galactosidase gene.