NOVELTY - An isolated polypeptide having xylanase activity is new.    USE - The polypeptide and composition are useful for degrading a cellulosic material, for producing a fermentation product (all claimed), for fermenting a cellulosic material or xylan-containing material, and for saccharifying the cellulosic material or xylan-containing material to fermentable sugars and to convert the fermentable sugars to useful fermentation products, e.g., fuel, potable ethanol, and/or platform chemicals (e.g., acids, alcohols, ketones, gases, and the like).    ADVANTAGE - Methods to improve cellulolytic and hemicellulolytic enzyme compositions through supplementation with additional enzymes to increase efficiency and to provide cost-effective enzyme solutions for degradation of lignocellulose are provided.    DETAILED DESCRIPTION - An isolated polypeptide having xylanase activity is selected from (a) a polypeptide having at least 95% sequence identity to the sequence of amino acids 18-349 of a defined sequence of 349 amino acids (SEQ ID NO. 2); (b) a polypeptide encoded by a polynucleotide that hybridizes under high stringency conditions with (i) the mature polypeptide coding sequence comprising a defined nucleotide sequence of 1386 bp (SEQ ID NO. 1), (ii) its cDNA sequence, or (iii) the full-length complement of (i) or (ii); and (c) a fragment of the sequence of amino acids 18-349 of SEQ ID NO. 2 that has xylanase activity, where the sequences are given in the specification. INDEPENDENT CLAIMS are:    (1) a process for degrading a cellulosic material by treating the cellulosic material with a composition comprising the polypeptide above, a cellobiohydrolase I, a cellobiohydrolase II, an endoglucanase, a beta -glucosidase, and a GH61 polypeptide; and    (2) a process for producing a fermentation product by (a) saccharifying a cellulosic material with a composition the polypeptide above, a cellobiohydrolase I, a cellobiohydrolase II, an endoglucanase, a beta -glucosidase, and a GH61 polypeptide; and (b) fermenting the saccharified cellulosic material with one or more fermenting microorganisms to produce the fermentation product.