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Tan spot, caused by Pyrenophora tritici-repentis (Ptr), is an economically important foliar disease in the major wheat growing areas of the world. Multiple races of the pathogen have been characterized based on their ability to cause necrosis and/or chlorosis in differential wheat lines. Isolates of race 5 cause chlorosis only, and they produce a host-selective toxin designated Ptr ToxB that induces chlorosis when infiltrated into sensitive genotypes. The international Triticeae mapping initiative (ITMI) mapping population was used to identify genomic regions harboring QTLs for resistance to fungal inoculations of Ptr race 5 and to determine the chromosomal location of the gene conditioning sensitivity to Ptr ToxB. The toxin-insensitivity gene, which we are designating tsc2, mapped to the distal tip of the short arm of chromosome 2B. This gene was responsible for the effects of a major QTL associated with …