Except for CMMRD-related neoplasms, tumours (adenomas and cancer) developed in the context of these particular syndromes are predominantly MMR-proficient, although a subset of tumours that develop in patients with biallelic PVs in MUTYH or PVs in POLE or POLD1 may show MMR deficiency as a result of biallelic somatic MMR gene mutations [103–105]. The gene discussed is MRC1; the disease is hyperinsulinemic hypoglycemia, familial, 4.