Using expressed sequence tag data, we obtained a full-length cDNA encoding a wheat protein inhibitor of xylanases (XIP-I). The 822 bp open reading frame encoded a protein of 274 amino acids with a molecular mass of 30.2 kDa, in excellent agreement with the native protein. Expression in Escherichia coli confirmed that the cDNA encoded a functional endo-1,4-beta-D-xylanase inhibitor. Its deduced amino acid sequence exhibited highest similarity to sequences classified as class III chitinases, but the inhibitor did not exhibit chitinase activity. This is the first full-length cDNA sequence that encodes a novel class of protein which inhibits the activity of endo-1,4-beta-D-xylanases.