Factor IX (F9) is a vitamin K-dependent serine protease that functions as a critical component of the intrinsic blood coagulation pathway. The protein circulates as an inactive zymogen and is activated by Factor XIa in the presence of calcium ions, undergoing proteolytic cleavage to generate Factor IXa, which then converts Factor X to its active form alongside Factor VIIIa, phospholipids, and calcium [PMID:659613]. The mature Factor IX protein contains a light chain originating from the amino-terminal region and a heavy chain from the carboxyl-terminal region, held together by disulfide bonds, with the catalytic triad located in the heavy chain 1. Factor IX deficiency causes hemophilia B (ChrX disease), an X-linked bleeding disorder occurring at approximately 1 in 10,000 males 2. Large deletions of F9 carry the highest risk of inhibitor formation in hemophilia B patients 3. Conversely, complete F9 duplication and triplication significantly elevate Factor IX activity and are associated with thrombotic phenotypes rather than bleeding 4. Hemophilia B Leyden represents a unique subset involving mutations in the F9 promoter that dynamically alter developmental gene expression 5. Multiple therapeutic approaches address factor IX deficiency, including recombinant factor IX proteins (such as nonacog beta pegol and eftrenonacogene dezaparvovec), prothrombin complex concentrates, and emerging gene therapies using CRISPR/Cas9 systems to enable endogenous factor IX production in patient cells 6.