Baseline serum concentrations of C3, C4, and CH100 were significantly lower in patients who later developed complications than in patients who were free of APS-related complications (86 ± 26 versus 107 ± 24 mg/dL (P = 0.042), 13 ± 7 versus 21 ± 8 mg/dL (P = 0.01), and 21 ± 17 versus 53 ± 16 U/mL (P = 0.007), resp). The gene discussed is C4A; the disease is autoimmune polyendocrinopathy.