An Open-Label, Non-Investigational Product, Multi-Center, Lead-In Study To Evaluate At Least 6 Months Of Prospective Efficacy And Selected Safety Data Of Factor IX (FIX) Prophylaxis Replacement Therapy In The Usual Care Setting Of Moderately Severe To Severe Adult Hemophilia B Subjects (FIX:C*2%) Who Are Negative For Neutralizing Antibodies (Nab) To Adeno-Associated Virus Vector (Aav)-Spark100.

1.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.;2.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).;3.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.;4.No known hypersensitivity to FIX replacement product.;5.No history of FIX inhibitor (clinical or laboratory-based assessment) defined as a titer 0.6 BU/mL, regardless of the laboratory normal range, or any measured Bethesda inhibitor titer greater than the upper limit of normal for the laboratory performing the assay. Clinically, no signs or symptoms of decreased response to FIX administration.;Subjects will not be required to undergo diagnostic evaluation of inhibitor status to participate in the study.

1.Anti-AAV-Spark100 neutralizing antibodies titer (above the lowest detectable titer) performed by a central laboratory during screening.;2. Lack of patient compliance with documentation of bleeds and/or FIX prophylaxis replacement therapy administration.;3. If there is no documentation regarding hepatitis status, as defined below, within the last 12 months prior to screening for hepatitis B and 6 months prior to screening for hepatitis C, then subjects will be required to have the following hepatitis testing performed at screening:;a. Hepatitis B screening (acute and chronic):;HBsAg (also referred to as Hepatitis B surface antigen), HBV-DNA viral assay (also referred to as a nucleic acid test for Hepatitis B virus DNA), and Anti-HBc (also referred to as Total Hepatitis B core antibody).;-A subject is not eligible if either HbsAg is positive or HBV-DNA is positive/detectable.;-Anti-HBc must be obtained in all subjects for determination of whether the subject had prior hepatitis B. If the anti-HBc is positive and both HBsAg and HBV DNA are negative this would be consistent with a prior infection and the subject would be eligible for the study. Anti-HBc must be obtained in all subjects to discriminate between those with no prior hepatitis B and those with prior infection in the event of reactivation. FDA has noted reactivation of hepatitis B virus exists.;-One documented negative HBV-DNA viral load is sufficient to assess eligibility. A subject who is currently undergoing anti-viral therapy for hepatitis B is not eligible.;b. Hepatitis C (acute or chronic):;-A subject who is currently undergoing anti-viral therapy for chronic hepatitis C is not eligible.;-Subjects treated with anti-viral therapy for chronic hepatitis C, must have completed anti-viral therapy at least 6 months prior to screening and have a negative HCV-RNA at least 6 months prior to screening.;-All subjects (who are not currently undergoing anti-viral therapy for chronic hepatitis C) must have a single HCV-RNA load assay (also referred to as a nucleic acid test [NAT] for HCV RNA) obtained during the 6 months preceding screening. This includes subjects with prior known chronic hepatitis C who have completed treatment with anti-viral therapy.;-A subject is not eligible if his HCV-RNA load assay result is positive/detectable.;4. Currently on antiviral therapy for hepatitis B or C.;5. 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.;All subjects who do not have the listed pre-existing diagnoses above must have the following assessments performed within the last 12 months prior to screening and if not will need to be tested for liver fibrosis status at screening:;-Measurement of serum albumin. 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.;6. 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. Subjects who are HIV positive and stable, have an adequate CD4 count (>200/mm3 ) and undetectable viral load (<50 gc/mL) documented within the preceding 12 months, and are on an antiretroviral drug regimen are eligible to enroll. Subjects who have not been tested within the prior 12 months of screening will need to be tested for HIV status at screening;7. Any patient who previously received SPK-9001 or any AAV gene based therapy.;8. Any patient with a planned surgical procedure requiring FIX surgical prophylactic factor treatment in the next 24 months.