While Peutz-Jeghers syndrome (PJS) has been acknowledged as a clinical entity for decades (1 ,2 ), the molecular background for the disease has been unraveled only very recently. PJS has two cardinal features: First, many but not all patients display mucocutaneous melanin pigmentation that is most prominently seen around the mouth, but can also be present for example in the buccal mucosa, lips, palms, feet, and in the anal region. Second, the patients have a predisposition to hamartomatous intestinal polyps. These lesions can occur anywhere in the gastrointestinal tract, but are most commonly seen in the small intestine (3 ). Tumor predisposition is not limited to intestinal hamartomas. The patients have a relatively unfocused increased risk of cancer, which has been reported to be 10- to 18-fold of that of the general population. Especially the relative risk for breast and gynecologic cancers is high (4 ,5 ). Other sites possibly involved include at least small and large intestine, and pancreas (3 ). Benign testicular tumors also occur commonly in the syndrome (6 ). Some of the malignant tumors may arise from the benign hamartomatous lesions, which appear to have some malignant potential at least in the context of PJS (7 -10 ).