PROS1 and antiphospholipid syndrome: Antiphospholipid autoantibodies (aPL antibodies) are a heterogeneous group of antibodies that underlie the pathogenesis of antiphospholipid syndrome via their interactions with phospholipid-binding plasma proteins such as β2GPI, prothrombin, thrombomodulin, plasminogen, antithrombin III, protein C, protein S, annexin II, annexin V, and likely others (22, 41–46).