ASS1 and hyperinsulinemic hypoglycemia, familial, 4: 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.