Technical Abstract: The gene Lr46 has provided slow rusting resistance to leaf rust caused by Puccinia triticina in wheat (Triticum aestivum) that has remained durable for almost 30 years. Using linked markers and wheat deletion stocks, we located Lr46 in the deletion bin 1BL 0.84-0.89 comprising 5% of the 1BL arm. The distal part of chromosome 1BL of wheat is syntenic to chromosome 5L of rice. Wheat expressed sequence tags (ESTs) mapping in the terminal 15% of chromosome 1BL with significant homology to sequences from the terminal region of chromosome 5L of rice were chosen for STS (sequence-tagged site) primer design and mapped physically and genetically. Also, sequences from two rice BAC clones covering the targeted syntenic region were used to identify additional linked wheat ESTs. Fourteen new markers potentially linked to Lr46 were developed; eight were mapped in a segregating population. Flanking markers were identified that were 2.0 cM proximal and 0.3 cM distal to Lr46. The physical location of Lr46 was narrowed to a sub-microscopic region between breakpoints of deletion lines 1BL-13 (FL=0.89-1) and 1BL-4 (FL=0.89-2). We are now developing a high resolution mapping population for positional cloning of Lr46.