ARG1 and hyperinsulinemic hypoglycemia, familial, 4: Arginase 1 Deficiency (ARG1-D; OMIM 207800) is a rare inherited metabolic disorder caused by biallelic pathogenic variants in the ARG1 gene (HGNC 663) impairing enzymatic activity of arginase 1 (ARG1, EC 3.5.3.1), the final step in the urea cycle.1