HCFC1 and hyperinsulinemic hypoglycemia, familial, 4: THcy may be, but is not consistently, elevated in haptocorrin deficiency, transcobalamin receptor deficiency (6, 7, 8), deficiency of a trifunctional enzyme (5,10-methylenetetrahydrofolate dehydrogenase, 5,10-methenyltetrahydrofolate cyclohydrolase, 10-formyltetrahydrofolate synthetase) in MTHFD1 deficiency (18), as well as in defects of the transcriptional co-regulator HCFC1 (19) or THAP11, a thanatos-associated protein (THAP) domain-containing transcription factor (20).