SLC6A8 and hyperinsulinemic hypoglycemia, familial, 4: Creatine Transporter Deficiency (CRTR-D) (MIM#300036) is a X-linked disorder caused by SLC6A8 gene mutation and represents the most frequent disorder of Creatine (Cr) metabolism [1-3] and one of the causes of X-linked mental retardation (XLMR) in a percentage varying from 2.1% to 3.5% in a European XLMR panel [4,5].