This study is for people with a genetic mutation in the coagulation (blood clotting) Factor 9 gene that causes severe or moderately severe hemophilia B. The main goal of this study is to learn if Regeneron Pharmaceuticals' gene therapy product, REGV131-LNP1265, improves patients with Hemophilia B's ability to form blood clots. Participants will receive a single dose of the study drug, given as an infusion into the vein. This treatment is considered experimental by the FDA. Regeneron Pharmaceuticals will also record if patients experience bleeds after gene therapy. If the participants do experience bleeds, the frequency of those bleeds will also be recorded.