Study of Recombinant Coagulation Factor IX Fc Fusion Protein, BIIB029, in Previously Treated Pediatric Participants With Hemophilia B

At the Baseline visit (28 ± 7 days prior to Day 1), participants receive a single IV
injection of prestudy FIX over 10 (±5) minutes in the clinic under medical supervision at a
dose of 50 IU/kg. A washout period with no FIX treatment is required prior to administration
of prestudy FIX and prior to rFIXFc. A PK assessment is done with prestudy FIX and also done
with rFIXFc on Day 1. After completing the PK assessments, participants begin weekly
prophylactic treatment with rFIXFc for approximately 50 weeks, to obtain 50 EDs. One ED is
defined as a 24-hour period in which a participant received 1 or more doses of rFIXFc, with
the time of the first injection of rFIXFc defined as the start of the ED.

Key Inclusion Criteria:

- Severe hemophilia B defined as ≤ 2 IU/dl (≤ 2%) endogenous FIX

- Male < 12 years and weight ≥ 13 kg

- History of at least 50 documented prior exposure days to FIX

- No history of, or currently detectable, inhibitor

Key Exclusion Criteria:

- Other coagulation disorders in addition to Hemophilia B

- History of anaphylaxis associated with any FIX or IV immunoglobulin administration

NOTE: Other protocol defined Inclusion/ Exclusion criteria may apply.