The present invention relates to protein engineering, and concerns especially family G/11 xylanases, and genes encoding said enzymes. In specific, the invention concerns Trichoderma reesei XYNII gene, which codes for endo-1,4-.beta.-xylanase (EC 3.2.1.8). The invention describes how site-directed mutagenesis can be used to improve the properties of an enzyme to match the industrial conditions where it is used. Protein engineering can be used to improve thermoactivity and thermostability of xylanases, as well as to broaden their pH range.