Somatic MMR testing was performed when indicative tumour molecular features were unexplained by LS or MLH1-hypermethylation: MSI-H MMR deficient (n = 8), MSS/MSI-L MMR deficient (n = 7), MSI-H MMR-proficient (n = 6), patchy MMR deficient (n = 18), and germline VUS_MMR (n = 1) cases (S4 Text). Here, MLH1 is linked to neoplasm.