β-Galactosidase (EC 3.2.1.23) is a glycoside hydrolase that hydrolyzes the β-glycosidic bonds formed between galactose and its organic moieties. In E.coli, the lacZ gene is the structural gene of β-galactosidase, which can be used as part of the induction system lac operon. β-Galactosidase can hydrolyze lactose to form glucose and galactose, and enter glycolysis; it can also catalyze the transgalactosylation of lactose into allolactose; allolactose can be cracked into monosaccharides^[1].