Six Month lead-in Study to Evaluate Prospective Efficacy and Safety Data of Current FIX Prophylaxis Replacement Therapy in Adult Hemophilia B Subjects (FIX:C≤2%)

Inclusion Criteria:

- Males ≥ 18 and < 65 years of age with moderate severe to severe hemophilia B and
documented FIX activity (≤2%) within the last 12 months prior to baseline visit.

- Willing and able to comply with scheduled visits, FIX prophylaxis treatment plan,
laboratory tests and other study procedures.

- Previous experience with FIX therapy (≥50 documented exposure days to a FIX protein
product such as recombinant, plasma-derived or extended half-life FIX product)

- Subjects as per usual care setting on FIX prophylaxis replacement therapy
(recombinant, plasma-derived or extended half-life FIX product) must have the
intention to remain on FIX prophylaxis replacement therapy for the duration of the
study

- No known hypersensitivity to FIX replacement product

- No history of FIX inhibitor (clinical or laboratory-based assessment) defined as a
titer ≥ to 0.6 BU/mL

Exclusion Criteria:

- Anti-AAV-Spark100 neutralizing antibodies titer (above the lowest detectable titer)
performed by a central laboratory during screening.

- Lack of patient compliance with documentation of bleeds and/or FIX prophylaxis
replacement therapy administration.

- A subject is not eligible if either HbsAg is positive or HBV-DNA is
positive/detectable.

- A subject who is currently undergoing anti-viral therapy for hepatitis B is not
eligible

- A subject who is currently undergoing anti-viral therapy for chronic hepatitis C is
not eligible.

- A subject is not eligible if his HCV-RNA load assay result is positive/detectable.

- Currently on antiviral therapy for hepatitis B or C.

- A subject is not eligible if any of the following pre-existing diagnoses, which are
indicative of significant underlying liver disease, are present in the medical record:

- Portal hypertension; or

- Splenomegaly; or

- Hepatic encephalopathy

- A subject is not eligible if the serum albumin level is below the testing laboratory's
lower limit of normal; and

- At least one of the following diagnostic tests for liver fibrosis indicating ≥
stage 3. The following results are indicative of fibrosis ≥ stage 3 and exclude
the subject from participation:

- FibroScan, with a score >8.3 kPa units;

- FibroTest/FibroSURE with a result >0.48; or

- AST-to-Platelet Ratio Index (APRI) >1.

- Documented serological evidence of human immunodeficiency virus HIV-1 or HIV-2 with
Cluster of Differentiation 4 positive (CD4+) cell count ≤200 mm3 within the last 12
months prior to screening.

- History of chronic infection or other chronic disease that the investigator deems an
unacceptable risk. Any patient with a history of thrombotic events including but not
limited to stroke or myocardial infarction. Any concurrent clinically significant
major disease or condition that the investigator deems unsuitable for participation.

- Participation in other studies involving investigational drug(s) within the last 3
months prior to study entry and/or during study participation or in a previous gene
therapy clinical study within the last 12 months prior to screening.

- Any subject with a planned surgical procedure requiring FIX surgical prophylactic
factor treatment in the next 24 months.