(USMLoid) A 55 year old woman comes to the clinic complaining of fatigue. Physical examination shows no abnormalities. A test for occult blood in the stool is positive. Her hematocrit is 30%. Her family history indicates that her mother her maternal grandfather, and a maternal aunt all died of colon cancer, each before the age of 60. Colonoscopy showed a mass in the proximal colon and a subsequent biopsy identified mucinous carcinoma. Karyotypes were normal. In which genes encoding components of the following functions is she most likely to bear an inherited mutation?