Deficiency of the FXIII-A subunit is responsible for the majority (~95%) of cases of congenital FXIII deficiency, and typically produces more severe clinical phenotypes due to loss of catalytic activity, whereas FXIII-B subunit defects mainly lower circulating heterotetramer levels by impairing stabilization and carriage.22–24. Here, F13B is linked to hyperinsulinemic hypoglycemia, familial, 4.