The microbial degradation of cellulose and xylans requires several types of enzymes such as endoglucanases (EC 3.2.1.4), cellobiohydrolases (EC 3.2.1.91) (exoglucanases), or xylanases (EC 3.2.1.8) [1,2]. Fungi and bacteria produces a spectrum of cellulolytic enzymes (cellulases) and xylanases which, on the basis of sequence similarities, can be classified into families. One of these families is known as the cellulase family F [3] or as the glycosyl hydrolases family 10 (GH10) [4,E1,E2]. All family 10 xylanases hydrolyze the glycosidic bond in a double-displacement 'retaining' mechanism using two catalytic acidic residues, where one residue acts a nucleophile (base) and the other acts as a general acid/base [6.7]. The enzymes which are currently known to belong to this family are listed below.

• Aspergillus awamori xylanase A (xynA).
• Bacillus sp. strain 125 xylanase (xynA).
• Bacillus stearothermophilus xylanase.
• Butyrivibrio fibrisolvens xylanases A (xynA) and B (xynB).
• Caldocellum saccharolyticum bifunctional endoglucanase/exoglucanase (celB). This protein consists of two domains; it is the N-terminal domain, which has exoglucanase activity, which belongs to this family.
• Caldocellum saccharolyticum xylanase A (xynA).
• Caldocellum saccharolyticum ORF4. This hypothetical protein is encoded in the xynABC operon and is probably a xylanase.
• Cellulomonas fimi exoglucanase/xylanase (cex).
• Clostridium stercorarium thermostable celloxylanase.
• Clostridium thermocellum xylanases Y (xynY) and Z (xynZ).
• Cryptococcus albidus xylanase.
• Penicillium chrysogenum xylanase (gene xylP).
• Pseudomonas fluorescens xylanases A (xynA) and B (xynB).
• Ruminococcus flavefaciens bifunctional xylanase XYLA (xynA). This protein consists of three domains: a N-terminal xylanase catalytic domain that belongs to family 11 of glycosyl hydrolases; a central domain composed of short repeats of Gln, Asn an Trp, and a C-terminal xylanase catalytic domain that belongs to family 10 of glycosyl hydrolases.
• Streptomyces lividans xylanase A (xlnA).
• Thermoanaerobacter saccharolyticum endoxylanase A (xynA).
• Thermoascus aurantiacus xylanase.
• Thermophilic bacterium Rt8.B4 xylanase (xynA).

The overall structure of the GH10 domain corresponds to an eightfold α/ β-barrel (TIM-barrel) with a typical deep groove in the centre, allowing an 'endo' type of action on the large polysaccharide backbone (see <PDB:1R85>) [6,7].

One of the conserved regions in these enzymes is centered on a conserved glutamic acid residue which has been shown [5], in the exoglucanase from Cellulomonas fimi, to be directly involved in glycosidic bond cleavage by acting as a nucleophile. We have used this region as a signature pattern. We have also developed a profile that covers the entire GH10 domain.