β-galactosidase is an exoglycosidase which hydrolyzes the β-glycosidic bond formed between a galactose and its organic moiety. It may also cleave fucosides and arabinosides but with much lower efficiency. It is an essential enzyme in the human body. Deficiencies in the protein can result in galactosialidosis or Morquio B syndrome. In "E. coli", the "lacZ" gene is the structural gene for β-galactosidase; which is present as part of the inducible system "lac" operon which is activated in the presence of lactose when glucose level is low. β-galactosidase synthesis stops when glucose levels are sufficient.