'''X-gal''' (also abbreviated '''BCIG''' for 5-bromo-4-chloro-3-indolyl-β-D-galactopyranosidegalactoside) is an [[organic compound]] consisting of [[galactose]] linked to a substituted [[indole]]. The compound was synthesized by [[Jerome Horwitz]] and collaborators in Detroit, MI, in 1964.<ref>Horwitz JP and 7 others, 1964. Substrates for cytochemical demonstration of enzyme activity. I. Some substituted 3-indolyl-β-D-glycopyranosides. Journal of Medicinal Chemistry 7: 574-575.</ref> The formal chemical name is often shortened to less accurate but also less cumbersome phrases such as bromochloroindoxyl galactoside. The X from [[indoxyl]] may be the source of the X in the X-gal contraction. X-gal is often used in [[molecular biology]] to test for the presence of an enzyme, [[beta-galactosidase|β-galactosidase]]. It is also used to detect activity of this enzyme in [[histochemistry]] and [[bacteriology]]. X-gal is one of many indoxyl glycosides and esters that yield insoluble blue compounds similar to [[indigo]] as a result of enzyme-catalyzed hydrolysis.<ref>Kiernan JA 2007. Indigogenic substrates for detection and localization of enzymes. Biotechnic & Histochemistry 82(2): 73-103.</ref>
