A galactoside is a compound formed from a simple sugar and another compound by replacing the hydroxyl group in the sugar molecule. The galactoside contains galactose (Wikipedia). β-galactosidase, also called lactase, beta-gal or β-gal, is a glycoside hydrolase enzyme that catalyzes the hydrolysis of β-galactosides into monosaccharides (any of the class of sugars (e.g. glucose) that cannot be hydrolysed to give a simpler sugar) through the breaking of a glycosidic bond. 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 (Wikipedia).