MRC1 and mismatch repair cancer syndrome 1: After Lynch syndrome or MLH1 hypermethylation was excluded as the cause of tumor MMR-deficiency, tumor and matched germline DNA sequencing identified a double (n = 119) or a single (n = 16) somatic MMR mutation.7 We hypothesized that 1 of these somatic MMR mutations may be a mosaic pathogenic variant for a proportion of these 135 participants.