NTHL1 heterozygotes do not appear to be at clinically significant increased risk for polyposis and/or CRC (Elsayed et al., 2020; NCCN Guidelines, 2022) and BC (Kumpula et al., 2020), but a slightly increased risk cannot be excluded (Li et al., 2021), as has been described for patients with heterozygous variants in the MUTYH gene (Win et al., 2014). This evidence concerns the gene NTHL1 and breast cancer.