Triticum aestivum xylanase inhibitor (TAXI)-type inhibitors are active against microbial xylanases from glycoside hydrolase family 11, but the inhibition strength and the specificity towards different xylanases differ between TAXI isoforms. Mutational and biochemical analyses of TAXI-I, TAXI-IIA andBacillus subtilisxylanase A showed that inhibition strength and specificity depend on the identity of only a few key residues of inhibitor and xylanase [Fierens K et al. (2005) FEBS J272, 5872–5882; Raedschelders Get al