The MET15 locus lies ∼250 kbp distal to the rDNA array, a series of ∼150–200 tandemly repeated copies of the genes encoding the structural RNA components of the ribosome [21]. To determine if the rDNA array represented a “hotspot” that was responsible for elevated LOH rates at MET15, we constructed a diploid strain that carried heterozygous markers immediately adjacent to each end of the rDNA array and at MET15 (Figure 1C). In aged populations, we isolated half-sectored colonies based on LOH at MET15 and scored each sector for the presence of the rDNA-linked markers on chromosome XII. We found that 90% of MET15 LOH events were linked to LOH events at the TRP1 marker distal to the rDNA array, but not the KANMX marker proximal to the array. This indicates that most MET15 LOH events originate within the rDNA array, with homozygosity extending ∼250 kbp from the rDNA array to the MET15 locus, and presumably the remaining ∼625 kbp to the telomere (Figure 1D). This LOH pattern is indicative of mitotic recombination events between homologous chromosomes. The fraction of mitotic
