This Phase 2 clinical research study is evaluating the safety and effectiveness of a once-daily oral investigational medicine that may improve anemia and reduce the need for red blood cell (RBC) transfusions among some patients who are chronically transfused. All participants enrolled in the study will receive the investigational medication.

We are currently enrolling individuals who meet the following criteria:*

• Regular RBC transfusions to prevent stroke or recurrence of stroke (Cohort A)
• Thalassemia and receiving regular RBC transfusions (Cohort B)
• Thalassemia and not receiving regular RBC transfusions (Cohort C)

There are additional eligibility requirements, which the investigator will explain to you.

This study is for people with a genetic mutation in the coagulation (blood clotting) Factor 9 gene that causes severe or moderately severe hemophilia B. The main goal of this study is to learn if Regeneron Pharmaceuticals' gene therapy product, REGV131-LNP1265, improves patients with Hemophilia B's ability to form blood clots. Participants will receive a single dose of the study drug, given as an infusion into the vein. This treatment is considered experimental by the FDA. Regeneron Pharmaceuticals will also record if patients experience bleeds after gene therapy. If the participants do experience bleeds, the frequency of those bleeds will also be recorded.