Of interest, reduced levels of plasma citrulline were found at newborn screening in 6 patients subsequently genetically diagnosed with MT-ATP6 variants and treated with citrulline supplementation that prevented metabolic crisis and reduced the severity of the disease.17 Therefore, citrulline might be considered both a biomarker and supportive treatment for patients with MT-ATPase deficiency. This evidence concerns the gene MT-ATP6 and hyperinsulinemic hypoglycemia, familial, 4.