FIX activity across time. (A) Cohort 1; (B) cohort 2. Only values at least 10 days after the preceding FIX concentrate administration, so that they are uncontaminated by exogenous FIX, are included. Participant 3 continued with prophylaxis after AMT-060 treatment so that only limited samples uncontaminated by exogenous FIX were available. The dotted line at FIX activity of 2 IU/dL indicates the threshold required for ceasing prophylaxis per protocol. FIX prophylaxis was continued after AMT-060 and tapered between week 6 and week 12. *Participant 4 had a moderate hemophilia B phenotype at baseline (FIX activity 1.5 IU/dL).