MRC1 and mismatch repair cancer syndrome 1: In particular, studies suggest up to 80% of cases suspected to have LLSs have double somatic MMR mutations (biallelic MMR deficiency) (Geurts-Giele et al. 2014; Haraldsdottir et al. 2014; Mensenkamp et al. 2014), with the most recent NCCN guidelines (Genetic/Familial High-Risk Assessment: Colorectal, version 1.2021) not available prior to the included studies, recommending screening for double somatic mutations in people with unexplained MMR deficiency.