X-LAG: How did they grow so tall?

X-linked acrogigantism (XLAG) is a new, pediatric-onset genetic syndrome, due to Xq26.3 microduplications encompassing the GPR101 gene. XLAG has a remarkably distinct phenotype with disease onset occurring before the age of 5 in all cases described to date, which is significantly younger than in other forms of pituitary gigantism. These patients have mixed GH and prolactin positive adenomas and/or mixed-cell hyperplasia and highly elevated levels of GH/IGF-1 and prolactin. Given their particularly young age of onset, the significant GH hypersecretion can lead to a phenotype of severe gigantism with very advanced age-specific height Z-scores. If not adequately treated in childhood, this condition results in extreme final adult height. XLAG has a clinical course that is highly similar to some of the tallest people with gigantism in history.