A Study to Investigate the Safety and Effectiveness of a Coagulation Factor IX Gene Insertion Therapy (REGV131-LNP1265) in Pediatric, Adolescent and Adult Participants With Hemophilia B

The study will be conducted with a 2-part adaptive design, with enrollment of patients into sequential parts of the study. Part 1: Dose Escalation and Dose Confirmation in adult patients ≥18 years of age * Dose Escalation Cohorts to determine the Recommended Dose for Expansion (RDE) of REGV131-LNP1265 * Dose Confirmation Cohort to gain further confidence in safety, tolerability, and Coagulation Factor IX (FIX) functional activity data at the RDE Part 2: Dose Expansion at the RDE * Part 2A: Adult patients ≥18 years of age: RDE of REGV131-LNP1265, as determined in Part 1 * Part 2B: Adolescent patients ≥12 to \<18 years of age will be administered weight-adjusted RDE * Part 2C: Adolescent and Pediatric patients ≥2 to \<12 years may be enrolled in an age staggered sequential manner; first participants aged ≥6 to \<12 years and then participants ≥2 to \<6 years of age and will receive a weight-adjusted RDE