MUTYH is a gene that helps to protect against polyps and gastrointestinal cancer (cancer of the bowel, small bowel and stomach). Everyone has two MUTYH genes (one from their mother, and one from their father). If both copies of the gene are not working, this is known as having two faulty MUTYH genes, or having biallelic (two) MUTYH mutations. People with two faulty MUTYH genes are known to have MAP.