Juvenile polyposis syndrome is caused by mutations in the bone morphogenetic protein receptor, type 1A (BMPR1A) or SMAD family member 4 (SMAD4), both of which are tumour suppressors.116–123 Cowden syndrome, results from mutations in PTEN. Homozygous mutations in the base excision repair (BER) pathway gene mutY DNA glycosylase (MUTYH) leads to MUTYH-associated polyposis syndrome, and heterozygous MUTYH mutations are observed in familial colorectal cancer. This evidence concerns the gene MUTYH and Cowden disease.