To explain the variable clinical phenotype in our AIPmut-positive patients, we evaluated the possible influence of two disease-modifying genes, GNAS1 and FGFR4. Whereas somatic GNAS1 mutations are common in unselected somatotropinomas (4.4%–59% of the cases) (28, –, 35), we have not identified any in adenomas from AIPmut-positive patients, suggesting that germline AIPmuts and somatic GNAS1 mutations are mutually exclusive in somatotropinomas. Here, FGFR4 is linked to growth hormone-producing pituitary gland neoplasm.