Haemophilia B, also spelled hemophilia B, is a blood clotting disorder causing easy bruising and bleeding due to an inherited mutation of the gene for factor IX, and resulting in a deficiency of factor IX. It is less common than factor VIII deficiency (haemophilia A).
Haemophilia B was first recognized as a distinct disease entity in 1952. It is also known by the eponym Christmas disease, named after Stephen Christmas, the first patient described with haemophilia B. In addition, the first report of its identification was published in the Christmas edition of the British Medical Journal.