Creatine deficiency syndromes (Figure 17): These include disorders of biosynthesis and transport of creatine, including guanidinoacetate methyltransferase deficiency (GAMT gene) [48] and L-arginine-glycine amidinotransferase deficiency (GATM gene) [49], and creatine transporter deficiencies (an X-linked disorder with SLC6A8 gene mutations) [17,50]. This evidence concerns the gene GAMT and hyperinsulinemic hypoglycemia, familial, 4.