That study found no difference in the prevalence of either the factor V Leiden mutation or the pro- thrombin 20210 mutation in patients with cryptogenic strokes compared with control subjects. The prevalence of the pro- 109) was higher in those with cryptogenic stroke and PFO versus those without PFO,827 suggesting a greater thrombotic risk in the setting of PFO than in either condition alone. The presumed stroke mechanism is paradoxical embolism related to venous rather than arterial thrombosis. Similarly, a familial cohort study found that the prothrombin 20210 mutation was a mild risk factor for venous thromboembolism but was not found to increase the risk of arterial thromboembolic events.828 Prothrombotic abnormalities have been identified in 20% to 50% of children with acute ischemic stroke and 33% to 99% of children with cerebral sinus venous thrombosis.829 In children with arterial ischemic stroke, emerging associations include an increased frequency of factor V Leiden mutation, elevated Lp(a), protein C deficiency, and aPL. The 2 most common genetic causes of thrombophilia are the factor V Leiden mutation and the G20210A mutation of Although their association with adult stroke is unclear, there is evidence of an association with ischemic stroke in children and young adults. A combination of mul- tiple case-control studies demonstrated an 4.3-fold increased incidence of factor V Leiden in children with acute ischemic A meta-analysis of the association of factorV Leiden with ischemic stroke in adults 50 years of age showed a fixed-effect OR of 2.00 for the mutation. 