SLEDAI-2K: Systemic Lupus Erythematosus Disease Activity Index 2000; low C3: low serum complement C3 (normal range: 0.9-1.8 g/L); low C4: low serum complement C4 (normal range: 0.1-0.4 g/L). The gene discussed is C3; the disease is systemic lupus erythematosus.