A hamartomatous polyp that occurs in the stomach, small and large intestines, and rarely within the esophagus, nasopharynx and the urinary tract. The Peutz-Jeghers polyps are grossly lobulated and dark. Microscopically, they have a central core of smooth muscle covered by mucosa. The smooth muscle shows tree-like branching. The question of whether or not the Peutz-Jeghers polyp is precancerous is a matter of controversy. The loss of heterozygosity on chromosome 19p (where the responsible gene LKB1 is located) suggests that the increased risk of malignancy may be due to malignant transformation from hamartoma to adenocarcinoma. --2002 [ NCIT:P378 ]