β-Galactosidase is a glycoside hydrolase enzyme that catalyzes hydrolysis of terminal non-reducing β-D-galactose residues in β-D-galactosides. β-Galactosides include carbohydrates containing galactose where the glycosidic bond lies above the galactose molecule. Substrates of different β-galactosidases include ganglioside GM1, lactosylceramides, lactose, and various glycoproteins. The β-galactosidase assay is used frequently in genetics, molecular biology, and other life sciences. An active enzyme may be detected using artificial chromogenic substrate 5-bromo-4-chloro-3-indolyl-β-D-galactopyranoside, X-gal. It is commonly used in molecular biology as a reporter marker to monitor gene expression. It is the most widely used biomarker for senescent and aging cells, because it is reliable and easy to detect.