Sequence exchange between PMS2 and its pseudogene PMS2CL, embedded in an inverted duplication on chromosome 7p22, has been reported to be an ongoing process that leads to functional PMS2 hybrid alleles containing PMS2- and PMS2CL-specific sequence variants at the 5′-and the 3′-end, respectively. The frequency of PMS2 hybrid alleles, their biological significance, and the mechanisms underlying their formation are largely unknown. Here we show that overall hybrid alleles account for one-third of 384 PMS2 alleles analyzed in individuals of different ethnic backgrounds. Depending on the population, 14-60% of hybrid alleles carry PMS2CL-specific sequences in exons 13-15, the remainder only in exon 15. We show that exons 13-15 hybrid alleles, named H1 hybrid alleles, constitute different haplotypes but trace back to a single ancient intrachromosomal recombination event with crossover. Taking advantage of an ancestral sequence variant specific for all H1 alleles we developed a simple gDNA-based polymerase chain reaction (PCR) assay that can be used to identify H1-allele carriers with high sensitivity and specificity (100 and 99%, respectively). Because H1 hybrid alleles harbor missense variant p.N775S of so far unknown functional significance, we assessed the H1-carrier frequency in 164 colorectal cancer patients. So far, we found no indication that the variant plays a major role with regard to cancer susceptibility. © 2010 Wiley-Liss, Inc.