STK11 and Peutz-Jeghers syndrome: Although LKB1 expression is exclusively cytoplasmic in lung and pancreatic cancer [57,58], gastrointestinal hamartomatous polyps from Peutz-Jeghers syndrome patients, head and neck squamous cell carcinoma, invasive lobular breast carcinoma, and solid papillary ductal carcinoma in situ breast cancer show both cytoplasmic and nuclear LKB1 expression [59-61].