A DELETERIOUS PMS2-PMS2CL HYBRID ALLELE 829 Figure 4. Possible mechanisms leading to the conﬁguration found in patient TR13: a deleterious hybrid PMS2 allele with no reciprocal hybrid PMS2CL allele. The normal conﬁguration depicted at the tops of panels A and B is that shown in Figure 3A (see legend of Fig. 3 for meaning of colors and symbols). (A) First hypothesis: Event I. Intra- chromosomal recombination with crossover generates chromosome A (Chr.A) containing both the deleterious hybrid PMS2 allele and a reciprocal hybrid PMS2CL allele. The recombination breakpoint is located in intron 10 (IVS10). Event II (occurring simultaneously with or after Event I). Interchromosomal recombination with crossover (the breakpoint is in the 0.7-Mb interduplicon sequence) between PMS2 allele and the nonhybrid PMS2CL allele segregate together, as shown by MLPA analysis of the proband’s mother, one of his maternal cousins, and the latter’s son (all mutation carriers) and four unaffected individuals, including the proband’s daughter and brother (data not shown). These two alleles are thus located on the same chromosome. This ﬁnding is compatible with two possible mechanisms of hybrid allele formation (Fig. 4). In the ﬁrst hypothesis (Fig. 4A), an initial intra- chromosomal recombination with crossover event generated the deleterious hybrid PMS2 allele. This allele was later separated from its reciprocal hybrid PMS2CL allele by an interchromosomal recombination event with a chromosome carrying a nonhybrid PMS2 allele and a nonhybrid Chr.A and Chr.B (characterized by a common inverted conﬁguration that is nondeleterious) gives rise to the conﬁguration observed in patient TR13 (Chr.C): the deleterious hybrid PMS2 allele and a nor- mal PMS2CL allele. (B) Second hypothesis: interchromosomal recom- bination event between Chr.B (as described in panel A) and a chromosome with the normal conﬁguration (Chr.E). With the recom- bination breakpoint located within intron 10, this sequence exchange generates the deleterious PMS2 hybrid allele found in patient TR13 (Chr.C). [Color ﬁgure can be viewed in the online issue, which is available at wileyonlinelibrary.com.] PMS2CL allele separated by a 0.7-Mb sequence whose orientation was inverted with respect to the normal conformation – a situation observed in at least 5% of all Caucasians (Feuk et al., 2005). If this mechanism generated the deleterious PMS2 hybrid allele, the interchromosomal recom- bination event must have taken place at least two generations before that of the proband and sev- eral generations after the intrachromosomal recombination event (more details below). Alter- natively, the two steps might have been com- pleted simultaneously, in a single, complex recombination event occurring at least one gener- ation before that of the proband’s mother. In the second hypothesis (Fig. 4B), there is only one step: an interchromosomal recombina- tion event between two chromosomes with PMS2 Genes, Chromosomes & Cancer DOI 10.1002/gcc