Human blood citrulline concentrations are potential biomarkers for various diseases, including sepsis (Kao et al. 2013; Wijnands et al. 2015), OTC deficiency (Lichter-Konecki et al. 2022), and citrullinemia (Quinonez and Lee 2022), which are caused by deficiencies in OAT, OTC, and argininosuccinate synthase (ASS), respectively. Here, OTC is linked to hyperinsulinemic hypoglycemia, familial, 4.