FXIII B-subunit, the carrier protein, is synthesized and secreted by the liver, whereas FXIII A-subunit is synthesized and released by cells of bone marrow origin. Both subunits are assembled in plasma to a tetrameric molecule consisting of 2 A- and 2 B-subunits (A₂B₂). In patients with congenital FXIII A-subunit deficiency, only free FXIII B-subunit circulates in plasma. After administration of rFXIII-A, both subunits are assembled in plasma to tetrameric FXIII in the same way as in healthy subjects.