The urea cycle is initiated by N‐acetyl glutamate synthase (NAGS), which catalyses the conversion of glutamate into N‐acetyl glutamate (NAG), an obligatory co‐factor of the first enzyme of the urea cycle, carbamoyl phosphate synthetase 1 (CPS1).57 Primary urea‐cycle disorders (UCDs) caused by defects in urea cycle enzymes, including NAGS deficiency, are characterised by altered levels of citrulline, ornithine, and arginine, depending on the site of the metabolic block. This evidence concerns the gene NAGS and hyperinsulinemic hypoglycemia, familial, 4.