Peutz-Jeghers syndrome (PJS), caused by germ-line mutations in LKB1, is characterized by the development of hamartomatous polyps in the gastrointestinal (GI) tract. McCune Albright syndrome (MAS), caused by somatic activating mutations in GNAS, presents with cutaneous, skeletal, and endocrine manifestations. Recently, hamartomatous GI polyps with histological features similar to those in PJS were observed in MAS patients, suggesting a role for GNAS in the pathogenesis of PJS. This study reports the first somatic GNAS mutation analysis in GI tumours of PJS patients. No mutations were observed, suggesting that GNAS is not involved in the pathogenesis of GI tumours in PJS.