PROS1 and systemic lupus erythematosus: Though rare in children, 40–50% of adult patients with SLE also produce an anti-phospholipid autoantibody, which inactivates prothrombin, protein C, and protein S.11 These are critical regulatory enzymes in the coagulation cascade, and their inactivation is responsible for the increased risk of bleeding and thromboembolic events found in patients with SLE.