The filamentous and saprophytic fungus Trichoderma reesei is a widely used microorganism in both research and industry. Its cellulose and hemicellulose degrading enzymes are applied in the pulp, paper, food, feed, textile industry and perhaps most importantly, in the production of second generation biofuels. Many T. reesei strains used in industry are descendants from the strain RUT-C30, in which the carbon catabolite repression is eliminated. Despite this, inducing substances are still needed to ensure a satisfactory protein formation. In one of these strains, a single point mutation in the fungal transcription factor middle homology region (FTFMHR) of the xylanase regulator 1 (Xyr1), leads to an expression of enzymes XynI and XynII in the presence of glucose, a normally repressing carbon source. Investigations show that under normal circumstances the carbon source triggers a conformational change of Xyr1 resulting in its activation or inactivation. Circular dichroism (CD) analyses of the mutated protein Xyr1A824V suggest that an altered secondary structure could hinder conformational change and thus cause the missing response of the protein in the presence of a carbon source. The aim of this work is to characterize the mutated strain and to identify and characterize the region around the mutation by looking for changes in the secondary structure of di-erent Xyr1 and Xyr1A824V protein fragments. As a main result a deregulation and strongly enhanced expression of the xylanase-encoding genes in the mutated strain, not resulting from a change in chromatin status, was revealed experimentally. Of nine Xyr1 and Xyr1A824V protein fragments, which were produced in this work, non showed a response to carbohydrates and only one showed a change in secondary structure. The latter also suggests that additionally to the mutation in the FTFMHR, the putative activation domain of Xyr1 is responsible for the previously observed changes in secondary structure. Analyses of the tertiary structure support this hypothesis.