A Study of Fitusiran (ALN-AT3SC) in Severe Hemophilia A and B Patients Without Inhibitors

Inclusion Criteria: * Males, \>=12 years of age. * Severe hemophilia A or B without inhibitors. * Severity confirmed by a central laboratory where FVIII level was less than (\<) 1 percent (%) or Factor IX (FIX) level was less than or equal to (\<=) 2% at Screening. * On-demand use of factor concentrate to manage bleeding episodes for at least the last 6 months prior to Screening, and meet each of the following criterion: * Nijmegen modified Bethesda assay inhibitor titer of \<0.6 Bethesda units per milliliter (BU/mL) at Screening. * No use of Bypassing agents to treat bleeding episodes for at least the last 6 months prior to Screening. * No history of immune tolerance induction therapy within the last 3 years prior to Screening. * A minimum of 6 bleeding episodes requiring factor concentrate treatment within the last 6 months prior to Screening. * Willing and complied with the study requirements and to provide written informed consent and assent. Exclusion Criteria: * Known co-existing bleeding disorders other than hemophilia A or B, i.e., Von Willebrand's disease, additional factor deficiencies, or platelet disorders. * Antithrombin (AT) activity \<60% at Screening. * Co-existing thrombophilic disorder. * Clinically significant liver disease. * Active hepatitis C virus infection. * HIV positive with a cluster of differentiation-4 count of \<200 cells/microliter. * History of arterial or venous thromboembolism. * Inadequate renal function. * History of mu…