Autoimmune FXIII deficiency due to an inhibitor (titers: 0.6-1.5 BU) accounted for 4.5% (2/44) of acquired FXIII deficiencies, and one resolved shortly after starting multiple myeloma treatment (a nadir FXIII:Act of 0.26 U/mL, normal FXIII-A:Ag on all samples; 0.6 BU inhibitor at presentation only). The gene discussed is RENBP; the disease is hyperinsulinemic hypoglycemia, familial, 4.