US and UK guidelines [12, 13] recommend that patients diagnosed with SLS be offered somatic genomic testing to identify the up to 70% of patients who have sporadic colorectal tumours with somatic inactivation of both copies of an MMR gene (referred to as “biallelic somatic MMR-deficiency”) [11, 14–18]. Here, MRC1 is linked to mismatch repair cancer syndrome 1.