Biallelic constitutional/germline loss-of-function NTHL1 variants confer predisposition to tumor formation demonstrating ‘COSMIC Signature 30’ mutation profile. Tumors have been reported at multiple primary sites; in particular adenomatous polyposis of colon (~10-50 polyps), colorectal cancer, and breast cancer. The disease is Monarch Disease Ontology identifier MONDO_0100502 (NTHL1-deficiency tumor predisposition syndrome).