Participants in this study have a genetic mutation, specifically in the coagulation (blood clotting) Factor 9 gene that causes severe or moderately severe hemophilia B. This study is researching an experimental gene insertion therapy (the adding of a gene into your DNA) called REGV131-LNP1265, also called the "study drug". Gene insertion therapy aims to teach the body how to produce clotting factor long-term, without the need for factor replacement therapy. The main aim of this study is to find a safe and well-tolerated dose of the study drug by checking the side effects that may happen from taking it. The study is looking at several other research questions including: * How much study drug is in the blood at different times * Whether the body makes antibodies against parts of the study drug, which could make the drug less effective or could lead to side effects. Antibodies are proteins produced by the body's immune system in response to a foreign substance * Whether the body makes antibodies against the clotting factor replacement therapy * How quality of life is affected by hemophilia B and if it changes after taking study drug * How joint health is affected by hemophilia B and if it changes after taking study drug * How often visits are required for the emergency room, urgent care center, physician's office, hospital, telephone or online are required as a result of bleeding events, and if the frequency changes after taking study drug * How often factor replacement therapy is needed, both on a regular basis for prevention of bleeding, and as needed to treat bleeding events (and it if changes after taking study drug) * Whether there is a difference in 2 different methods for measuring Factor 9 activity in the blood