Mutations in the F9 gene cause factor IX (FIX) deficiency, leading to severe bleeding in people with hemophilia B. Current therapies rely on frequent FIX infusions or newer agents that rebalance coagulation, but both approaches still require weekly to bimonthly dosing and often fail to entirely prevent breakthrough bleeding events, which remain common in patients. Therefore, new therapeutic strategies providing a predictable, durable, active and redosable source of FIX are urgently needed.