It has been shown [1,2,E1] that the following glycosyl hydrolases can be, on the basis of sequence similarities, classified into a single family:

• β-galactosidases (EC 3.2.1.23) from bacteria such as Escherichia coli (genes lacZ and ebgA), Clostridium acetobutylicum, Clostridium thermosulfurogenes, Klebsiella pneumoniae, Lactobacillus delbrueckii, or Streptococcus thermophilus and from the fungi Kluyveromyces lactis.
• β-glucuronidase (EC 3.2.1.31) from Escherichia coli (gene uidA) and from mammals.

One of the conserved regions in these enzymes is centered on a conserved glutamic acid residue which has been shown [3], in Escherichia coli lacZ, to be the general acid/base catalyst in the active site of the enzyme. We have used this region as a signature pattern. As a second signature pattern we selected a highly conserved region located some sixty residues upstream from the active site glutamate.