Considering C4 is usually analyzed by FIA-MS/MS (flow injection analysis–tandem mass spectrometry) in newborn screening programs, and chromatographic separation is not used, C4 could also correspond to isobutyrylcarnitine, a biomarker of isobutyryl-CoA dehydrogenase deficiency (OMIM 611238) in the valine metabolism. The gene discussed is ACAD8; the disease is hyperinsulinemic hypoglycemia, familial, 4.