Hemophilia B (Haemophilia B, Hæmophilia B, Christmas disease) is a blood clotting disorder caused by a mutation of the Factor IX gene. It is the second most common form of hemophilia, rarer than hemophilia A. It is sometimes called Christmas disease after Stephen Christmas, the first patient described with this disease. In addition, the first report of its identification was published in the Christmas edition of the British Medical Journal. Treatment (bleeding prophylaxis) is by intravenous infusion of factor IX.
The factor IX gene is located on the X chromosome (Xq27.1-q27.2). It is inherited X-linked recessive, which explains why - as in hemophilia A - only males are generally affected.
Factor IX deficiency leads to an increased propensity for hemorrhage. This is in response to mild trauma or even spontaneously, such as in joints (hemarthrosis) or muscles.
Source: Wikipedia