Patients with guanidinoacetate methyltransferase (GAMT) deficiency have much higher guanidinoacetate concentrations in brain (3.4-3.6 mmol/L) (Stockler et al. 1994) and CSF (11–12 μmol/L) (Stockler-Ipsiroglu et al. 2014). The gene discussed is GAMT; the disease is hyperinsulinemic hypoglycemia, familial, 4.