This study is for boys aged 12 to 18 who have severe *hemophilia B*, a condition where blood doesn't clot properly due to low levels of a protein called *FIX*. The study tests a new treatment called **CSL222**. Participants must have been using regular *FIX* treatments for at least two months and have had 75 previous treatments. The study checks the safety and effectiveness of CSL222, which is a single dose given once. **Key Points:** - The study lasts at least 6 months before treatment and requires prior *FIX* treatments. - Participants should have no history of certain medical issues, like liver problems or other bleeding conditions. - The study involves multiple visits to the study center and requires specific blood tests. This study is called "open-label," meaning both doctors and participants know they are receiving CSL222, not a placebo. Before joining, participants must meet the eligibility criteria, and their health will be closely monitored throughout the study.