SSc patients with PAH had similar CH50 [79.24 U/mL (IQR 74.05–110.85 U/mL) vs. 75.86 U/mL (IQR 65.46–87.68 U/mL); p > 0.05], C2 [25 mg/L (IQR 22.4–25.7 mg/L) vs. 27 mg/L (24.1–32.3 mg/L); p > 0.05], C3 [1.13 g/L (IQR 0.85–1.26 g/L) vs. 1.13 g/L (IQR 1.01–1.23 g/L); p > 0.05], C4 [0.19 g/L (IQR 0.16–0.28 g/L) vs. 0.2 g/L (IQR 0.16–0.23 g/L); p > 0.05] than SSc patients without PAH. The gene discussed is C2; the disease is pulmonary arterial hypertension.