A type of familial infantile gigantism caused by microduplication of Xq26.3. Onset usually occurs in the first year of life in previously normal infants. Patients present with gigantism and may associate acromegalic features (e.g. coarse facial features, frontal bossing, prognathism, increased interdental space) as well as marked enlargement of hands and feet, soft tissue swelling, appetite increase and acanthosis nigricans. May present as a sporadic condition or as familial isolated pituitary adenomas.