A 50-year-old woman who has a family history of breast cancer presents with a 6-month history of increasing abdominal girth. On close questioning, she volunteers a history of vague abdominal pain dating back 1 year. She has no children and has never been pregnant. Bimanual pelvic examination reveals a 10-cm right adnexal mass. Percussion of the abdomen indicates ascites. Aspiration cytology of the ascites fluid reveals malignant papillary structures with psammoma bodies. A mutation in which of the following genes is most likely associated with this patient's malignant disease?