Hemophilia B results from pathogenic variants in the F9 gene, located at chromosomal locus Xq27.1, which encodes coagulation factor IX—a serine protease essential to the intrinsic clotting cascade. The condition follows X-linked recessive inheritance. GeneReviews confirms that all males with an F9 pathogenic variant manifest hemophilia B at a severity consistent with other affected males in the family, though modifying genetic and environmental effects may alter clinical expression to some extent. Per GeneReviews genotype-phenotype correlations, large deletions, nonsense variants, and most frameshift variants cause severe disease, while missense variants produce a range of severity from severe to mild depending on their location and the specific substitution involved. Alloimmune inhibitor development—neutralizing antibodies against infused factor IX—is a clinically significant complication documented in a subset of individuals with severe disease. Germline mosaicism has been observed and can result in transmission from a parent who does not carry the variant in somatic tissue.

Diagnosis of hemophilia B is established by measurement of factor IX clotting activity. Per GeneReviews, factor IX clotting activity below 40% of normal, combined with a normal prothrombin time (PT) and prolonged activated partial thromboplastin time (aPTT), establishes the diagnosis. Severity thresholds are: severe (below 1%), moderately severe (1–5%), and mild (5–40%). Confirmatory molecular genetic testing of the F9 gene identifies the specific pathogenic variant, supporting carrier determination in at-risk female relatives. The differential diagnosis includes hemophilia A (factor VIII deficiency) and other inherited bleeding disorders, which are distinguished by specific factor assays. GeneReviews notes that the diagnostic use of "male" and "female" in this condition refers narrowly to biological sex for the purposes of X-linked inheritance. Family history and laboratory evaluation constitute the primary diagnostic pathway in most clinical settings.

Multiple FDA-approved therapies are documented for hemophilia B. Plasma-derived coagulation factor IX (Human) products include Mononine and Alphanine SD. Recombinant coagulation factor IX products include BeneFix, Rixubis, and Ixinity, all with ACTIVE market status. Two gene therapy products have received FDA approval: Hemgenix (etranacogene dezaparvovec-drlb), approved November 2022, delivering a functional F9 gene via an AAV5 vector; and Beqvez (fidanacogene elaparvovec-dzkt), approved April 2024, delivering the high-activity Padua variant of factor IX. Non-factor rebalancing therapies that received FDA approval include Alhemo (concizumab-mtci), an anti-TFPI antibody approved December 2024; Hympavzi (marstacimab-hncq), a TFPI inhibitor approved October 2024; and Qfitlia (fitusiran), an RNA interference therapy targeting antithrombin, approved March 2025. The World Federation of Hemophilia has published treatment guidelines referenced in GeneReviews, and formal clinical practice guidelines for hemophilia B are established. GeneReviews documents that individuals with hemophilia followed at hemophilia treatment centers have lower mortality than those not receiving center-based care. Certain clinical circumstances are documented in GeneReviews as warranting attention in hemophilia B, including the need for factor IX assessment prior to circumcision procedures in males with known family history.

The clinical course of hemophilia B is substantially shaped by disease severity and access to treatment. Per GeneReviews, untreated or inadequately treated severe hemophilia B leads to recurrent hemarthrosis and progressive hemophilic arthropathy with joint disability. With prophylactic factor IX replacement, individuals with severe hemophilia B experience markedly reduced rates of spontaneous hemarthrosis compared to on-demand treatment approaches. Gene therapy with Hemgenix and Beqvez has demonstrated sustained expression of factor IX at clinically meaningful levels in treated individuals, with some achieving factor IX activity in the mild or normal range; however, GeneReviews notes that long-term durability and safety of these approaches require further study. The symptomatic form in female carriers varies in severity depending on X-inactivation patterns and residual factor IX levels. Surveillance data cited in GeneReviews confirm that hemophilia treatment center-based care is associated with reduced mortality.

Numerous active clinical trials for hemophilia B are registered on ClinicalTrials.gov, reflecting robust ongoing investigation across multiple therapeutic platforms. Documented active studies include efficacy and safety evaluation of etranacogene dezaparvovec (CSL222), lentiviral FIX gene therapy approaches, and the ATHN Transcends natural history study of non-neoplastic hematologic disorders. Per GeneReviews, gene therapy for hemophilia B remains a highly active area, with additional studies of gene therapy focusing on long-term durability and safety of already-approved products and novel investigational vectors. Research is also ongoing in gene editing approaches combining viral vectors with lipid nanoparticle delivery of Cas9 components, as reflected in active orphan designations for investigational agents. The World Federation of Hemophilia maintains an international patient registry supporting natural history research.