F2 and Hermansky-Pudlak syndrome: This phenomenon that increased the inhibitor activity by a normal plasma component was called “lupus cofactor” (LC).6, 7Loeliger suggested that responsible for the (unknown) “cofactor” could be prothrombin (PT), while others said that LC was driven by β2‐glycoprotein I (β2‐GPI).8In this report, we describe a patient with LA-HPS caused by circulating antibodies against PT and prove that prothrombin is responsible for the observed LC phenomenon.