A 26-year-old healthy man comes to your clinic for an annual wellness examination. He does not take any medications. He smokes half pack of cigarettes daily. He tells you that his father died of colon cancer at the age of 45. He also has a 25-year-old cousin who recently had colonoscopy for rectal bleeding was found to have multiple polyps and is scheduled for total colectomy. Your patient wants to know if he can inherit colon cancer and if there is a way to find out if he is at risk. You talk to him about how some cancers can be caused by genetic mutations. For what genetic mutation is this patient at highest risk?