Study design.^aAfter the administration of AMT-060 to each of the first 2 participants in each cohort, the Data Monitoring Committee evaluated available safety data during 24 hours before dosing of the next participant could be initiated. ^bProphylactic FIX replacement therapy was generally tapered between weeks 6 and 12 if FIX activity was ≥2.0 IU/dL in at least 2 consecutive visits. Investigators could taper prophylaxis later than 12 weeks at their discretion. The decision to continue tapering/withholding of prophylactic FIX replacement therapy was based on the individual assessment by the investigator but included the requirement to document that the participant could maintain a FIX activity level ≥2.0 IU/dL. ^cCohort 2 dosing was initiated after the completion of cohort 1 dosing and review of initial safety data by the Data Monitoring Committee.