Individuals with homozygous or compound heterozygous likely pathogenic or pathogenic (LP/P) variants in the base excision repair genes MUTYH [1] and NTHL1 [2] (i.e., biallelic cases) predispose to the development of multiple pre-cancerous adenomas in the colon (adenomatous polyposis), CRC and a spectrum of extra-colonic cancers [2,3]. The gene discussed is NTHL1; the disease is adenoma.