Newly published research using whole-exome sequencing has identified a new form of familial adenomatous polyposis that occurs in individuals who are homozygous for loss-of-function mutations in the base-excision repair gene NTHL1. This loss of function arises from a range of mutations, including c.268C>T and c.391C>T. In total, 51 individuals from three families with a family history of adenomatous polyposis, were selected for this study. Seven individuals from the three families had various cancers, including colorectal cancer. All patients with cancer were homozygous for NTHL1 loss-of-function mutations, unaffected family members were either heterozygous or did not carry mutant NTHL1. These findings suggest that NTHL1 is a susceptibility gene for recessive adenomatous polyposis and possibly other types of cancer.