Key biochemical findings were elevated guanidinoacetate in urine in the two GAMT deficiency patients (803 and 1912 mmol/mol creatinine, reference 15–152) and increased urinary creatine in the four CRTR patients (5280, 4900, 3110, and 4890 mmol/mol creatinine, respectively, reference 19-1046). Here, GAMT is linked to hyperinsulinemic hypoglycemia, familial, 4.