This is an open- label, non- randomized, uncontrolled, dose-escalation pilot study to evaluate the safety and efficacy of KL001 injection solution in hemophilia B subjects with ≤2 IU/dl residual FIX levels.

This is an open- label, non- randomized, uncontrolled, dose-escalation pilot study to evaluate the safety and efficacy of KL001 injection solution in hemophilia B subjects with ≤2 IU/dl residual FIX levels. Nine subjects will be enrolled and administered with three different doses of KL001. Subjects will provide informed consent and then undergo screening assessments up to 1 month prior administration of KL001. All subjects will undergo 52 weeks safety observation and will be encouraged to enroll in an extension study to evaluate long- term safety of KL001 for total 5 years.

Inclusion Criteria:

1. Male ≥12 years of age.
2. Moderate/severe or severe hemophilia B (baseline FIX activity ≤2% of normal or documented history of FIX activity ≤2%).
3. At least 3 bleeding episodes per year that require on-demand treatment with FIX or are treated with a prophylactic regimen of FIX.
4. At least 100 days exposure history to recombinant or plasma-derived FIX protein products.
5. No neutralizing antibodies to exogenous FIX protein products.
6. Willing and able to comply with study procedures and requirements.

Exclusion Criteria:

1. Suffering from chronic inflammatory muscle disease.
2. Positive in Hepatitis B or Hepatitis C.
3. Infection with HIV-1 or HIV-2 and CD4+T cell count ≤ 200/ μ L.
4. History of thrombosis or susceptibility to thrombosis.
5. Current or previous participation in another gene therapy study.
6. Has any other significant medical condition that the investigator feels would be a risk to the subject or would impede the study.