Used as a chromogenic substrate in molecular biology for the detection of α-galactosidase enzyme activity. When the enzyme cleaves the glycosidic bond, it releases the 6-chloro-3-indolyl group, which oxidizes to form a blue insoluble precipitate. This reaction is commonly employed in bacterial colony screening, especially in gene expression studies and plasmid cloning where α-galactosidase acts as a reporter enzyme. The color change allows easy visual identification of positive clones without the need for advanced instrumentation.