STK11 and Peutz-Jeghers syndrome: The discovery of LKB1 as the tumor suppressor gene responsible for Peutz-Jegher syndrome, an autosomal-dominant disorder characterized by melanocytic macules of the lips, multiple gastrointestinal hamartomatous polyps, and an increased risk for various neoplasms including gastrointestinal cancer led to the suspicion that metformin may exhibit antitumor properties, because LKB1 is an upstream AMPK regulator [264].