X-gal (5-bromo-4-chloro-3-indolyl-β-D-galactoside), a histochemical substrate for β-galactosidase, yields a blue precipitate upon hydrolysis, making it suitable for use in immunoblotting or immunocytochemical assays. Bacteria containing active β-galactosidase produce blue colonies when grown on plates containing X-gal. X-gal is supplied as white crystals and is soluble in dimethylformamide.