MRC1 and mismatch repair cancer syndrome 1: MMRDetect analysis using WES data revealed that 29.4% of MMR-altered breast cancers exhibited MMR deficiency (Table S2), a proportion significantly higher than that observed in MMR-wt breast cancers (5/17 vs. 2/676, p < 0.001, Chi Square test) (Table 1), further supporting that somatic MMR alterations led to impaired function of DNA damage repair in cancer cells, particularly the MMR pathway.