X-Gal

X-gal (5-bromo-4-chloro-3-indolyl-ß-D-galactopyranoside or BCIG) is an analog of lactose and a chromogenic substrate for ß-galactosidase. In the presence of the enzyme, X-gal is hydrolyzed into galactose and a final product which turns intensely blue. The X-gal and galactose system is used in applications involving functional ß-galactosidase as reporters, such as blue/white genetic screening and two-hybrid interactions.