F2 and autoimmune polyendocrinopathy: Finally, we analyzed the SN-APS sera for the presence of antibodies against two major cofactor proteins for aPL, prothrombin and annexin V. Our results revealed that 3 out of 24 sera from SN-APS patients (12.5%) displayed antibodies to prothrombin and 1 (4.2%) to annexin V. The datum of antiprothrombin might be underestimated, since we tested anti-prothrombin antibodies in the absence of phosphatidylserine.