RPL patients had elevated levels of complement C4 and ACLA-IgG, and decreased concentration of factor H. Remarkably, female aPL-positive subjects with homozygous C4B deficiency were all protected from RPL, which is consistent with observations in mouse models that complement C4 deficiency or C3 deficiency were protective from RPL induced by injection of human aPL (16). Here, C4B is linked to hyperinsulinemic hypoglycemia, familial, 4.