X-LAG is a rare syndrome of very early childhood onset pituitary tumors/hyperplasia that leads to growth hormone excess and severe overgrowth and pituitary gigantism. Three FIPA families with X-LAG have been reported to date all of which had transmission of a chromosome Xq26.3 duplication from affected mother to affected son. The disease characteristics of very young onset pituitary gigantism leads to severe overgrowth if not treated adequately; many of the tallest humans in history (e.g. Robert Pershing Wadlow; Sandy Allen, André Rousimoff (Andre the Giant), Zeng Jinlian) had a similar clinical history to patients with X-LAG syndrome. The tallest historical individual with a known genetic cause was Julius Koch (Geant Constantin) who was found to have X-LAG on genetic study of his skeleton. X-LAG has 100% penetrance so far (all affected with the Xq26.3 duplication have the disease and it affects predominantly females. Isolated non familial cases of X-LAG can either have a constitutional duplication of a chromosome Xq26.3 including "GPR101", or mosaicism for the duplication (present in a minority of cells) in the case of isolated male patients. X-LAG causes about 10% of cases of pituitary gigantism.