We presented positive interim clinical data from our ongoing Phase 1 study for ALN-AT3, a subcutaneously administered, investigational RNAi therapeutic targeting antithrombin (AT) for the treatment of hemophilia and rare bleeding disorders. Data were presented at the International Society on Thrombosis and Haemostasis (ISTH) 2015 Congress held June 20 – 25, 2015. New clinical results from 12 subjects with severe hemophilia show that subcutaneous administration of ALN-AT3 achieved potent and dose-dependent knockdown of AT of up to 86%. AT knockdown was highly durable, with effects lasting over two months after the last dose, supporting further evaluation of a once-monthly subcutaneous dose regimen. In addition, AT knockdown was associated with statistically significant increases in thrombin generation with a mean increase of up to 350% and marked improvements in whole blood clotting; these results demonstrate a re-balancing of hemostasis in severe hemophilia subjects. Furthermore, in an exploratory post-hoc analysis, a reduced frequency of bleeding was observed at higher AT knockdown levels with a maximum bleed-free interval of 114 days. Very importantly, ALN-AT3 continues to be generally well tolerated.