Inclusion Criteria: * Adult Males (≥18) with moderately severe to severe Hemophilia B (FIX deficiency) * Received ≥50 exposure days to Factor IX products preceding enrollment. * Currently receiving prophylaxis treatment * Adequate organ function and clinical labs * Able to tolerate study procedures including leukapheresis. Exclusion Criteria: * Pre-existing or history of specific diseases * B-Cell malignancy, EBV lymphoproliferative disease * Primary immunodeficiency disease or disorder (PIDD) or systemic immuno-suppression * Arterial and/or venous thromboembolic events within 2 years prior to dosing * History of anaphylaxis or nephrotic syndrome * Active infection (HIV, Hep B or C) * History of inhibitor to FIX or inhibitor * History of an allergic reaction or anaphylaxis to FIX products * Planned surgical procedure within 6 months from BE-101 administration * Previously dosed with gene therapy * Participated in an interventional study and/or received an interventional study drug within 30 days or five-half-lives (whichever is longer) of consent into BeCoMe-9 and for the duration of the study * Planned participation in clinical trial within one year after BE-101 * Administration of an investigational agent or vaccine within 28 days of leukapheresis and dosing of BE-101 Other protocol-defined inclusion/exclusion criteria may apply.