Arginase 1 deficiency (ARG1‐D; OMIM # 207800), also known as hyperargininaemia, is an ultrarare progressive, autosomal recessive, metabolic disease, affecting approximately 1 in 726 000 to 950 000 births.1, 2. Here, ARG1 is linked to hyperinsulinemic hypoglycemia, familial, 4.