In the three families described in our study, we were able to identify nine pathogenic NTHL1 biallelic variant carriers (homozygous or compound heterozygous), of which seven were patients that had polyposis diagnosed before genetic testing and two were relatives that, after a positive pre-symptomatic genetic testing, presented colorectal polyps in the first colonoscopies. The gene discussed is NTHL1; the disease is polyp of large intestine.