In bread wheat, the Pm3 locus encodes one of the earliest identified allelic series of resistance genes against a wheat disease. Ten Pm3 alleles (Pm3i to j) have been genetically identified in wheat lines or cultivars originating from four continents. Each of them confers race-specific resistance to the fungal pathogen wheat powdery mildew (Blumeria graminis f. sp. tritici). Given the fact that the wheat genome is large (1.7 x 1010 bp), hexaploid with three homoeologous genomes and contains a high amount of repetitive DNA, comparative genomics is an essential tool for map-based gene cloning in bread wheat. In our group we are using comparative genomic approaches to isolate resistance genes against the fungal diseases powdery mildew and leaf rust. We have used the molecular tools developed for the isolation of the known alleles to screen for new Pm3 alleles in the gene pool of tetraploid and hexaploid wheat. From the later, a set of more than 1300 landraces was screened for powdery mildew resistance and the presence of a Pm3 haplotype. Most lines contained a susceptible Pm3 allele which is closely related to the functional Pm3 resistance genes. We have also identified resistant lines with new types of Pm3 allelic sequences. These new alleles are currently tested for activity as resistance genes. The diversity and functionality of these genes will be presented.