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Germline mutations of the DNA mismatch repair genes MLH1, MSH2, MSH6 or PMS2, and deletions affecting the EPCAM gene adjacent to MSH2, underlie Lynch syndrome by predisposing to early-onset colorectal, endometrial and other cancers. An alternative but rare cause of Lynch syndrome is constitutional epimutation of MLH1, whereby promoter methylation and transcriptional silencing of one allele occurs throughout normal tissues. A dominantly transmitted constitutional MLH1 epimutation has been linked to an MLH1 haplotype bearing two single-nucleotide variants, NM_000249.2: c.−27C>A and c.85G>T, in a Caucasian family with Lynch syndrome from Western Australia. Subsequently, a second seemingly unrelated Caucasian Australian case with the same MLH1 haplotype and concomitant epimutation was reported. We now describe three additional, ostensibly unrelated, cancer-affected families of European heritage with this MLH1 haplotype in association with constitutional epimutation, bringing the number of index cases reported to five. Array-based genotyping in four of these families revealed shared haplotypes between individual families that extended across ≤2.6-≤6.4 megabase regions of chromosome 3p, indicating common ancestry. A minimal ≤2.6 megabase founder haplotype common to all four families was identified, which encompassed MLH1 and additional flanking genes and segregated with the MLH1 epimutation in each family. Our findings indicate that the MLH1 c.−27C>A and c.85G>T variants are borne on a European ancestral haplotype and provide conclusive evidence for its pathogenicity via a mechanism of epigenetic silencing of MLH1 within normal tissues. Additional descendants bearing this founder haplotype may exist who are also at high risk of developing Lynch syndrome-related cancers. [ABSTRACT FROM AUTHOR]