-galactosidase (-gal) catalyzes the conversion of X-gal to galactose and 5-bromo-4-chloro-3-hydroxyindole. The functionality of -gal is indirectly measured by the blue precipitate of dimeric 5-bromo-4-chloro-3-hydroxyindole in the presence of potassium ferricyanide or ferrocyanide. The colorimetric detection methods also use other substrates of -gal like O-nitrophenyl--D-galactopyranoside (ONPG). -gal is a cost-effective simple and standard reporter system. The Galactosidase Reproter Gene Staining Kit provides a method for determining the percentage of cells transfected with the plasmid expressing the -galactosidase gene LacZ. It can also be used to visualize the specific expression of the reporter gene in tissue sections by microscopy.