In fact, some of us [13] have even suggested that guanidinoacetic acid may be useful in GAMT deficiency at the muscular level because it is phosphorylated to phospho-guanidinoacetic acid and sets up a guanidinoacetate–phosphoguanidinoacetate system that may replace the creatine–phosphocreatine system when the latter is missing. This evidence concerns the gene GAMT and hyperinsulinemic hypoglycemia, familial, 4.