This study is focused on males who have Hemophilia B and who need regular preventive treatment with factor IX protein (FIX) replacement therapy to prevent and also to control their bleeding events. The aim of the study is to gather at least 6 months of information on bleeding events for each individual participant while they continue to use their usual FIX replacement therapy. There is no experimental treatment being tested in this study. The study is informational, and part of a larger program to understand and treat Hemophilia B with a potential experimental new therapy in the future. There is no obligation to agree to taking part in this future study. The study is looking to answer several other research questions to help understand each participant's individual disease characteristics, including: * How often to use FIX replacement therapy, both on a regular basis (prophylaxis) and as needed to treat bleeding events * Measurement of FIX activity (factor IX is a clotting factor) by different laboratories using different types of tests in Hemophilia B participants * Possible complications from the FIX replacement therapy the patient receives (usual standard of care will continue to be used) * How quality of life is affected by Hemophilia B * How joint health is affected by Hemophilia B * How often the participant visits the emergency room, urgent care center, physician's office, hospital, or has a telemedicine visit as a result of bleeding events * Whether the body makes antibodies (a protein produced by the body's immune system) against the FIX replacement therapy you receive, which could make the drug less effective or could lead to side effects