In 19/324 patients (5.9%), thrombophilia was recognized as the underlying cause of stroke: antiphospholipid syndrome was present in ten patients, mutations in the factor V Leiden or prothrombin genes were detected in four patients, a deficiency of protein S was observed in one patient, and hyperhomocysteinemia (persistently elevated plasma levels >20 μmol/L measured at admission and after three months) was observed in four patients. The gene discussed is PROS1; the disease is Rare hereditary thrombophilia.