|Factor IX and haemophilia B||
Haemophilia is a disorder of the blood-clotting system. Normally when you cut yourself your blood plugs the wound by changing from a liquid to a solid and forming a blood clot.
If you have haemophilia your blood will not clot normally and you may bleed for longer than normal, or you may bleed internally, particularly into joints such as your knees, elbows and ankles. Blood contains special proteins, called clotting factors, which are involved in the clotting process. There are 20 different clotting factors. If you have haemophilia, your body does not produce enough, or produces none of these clotting factors.
There are different types of haemophilia. If you have haemophilia B, you have a deficiency of clotting factor IX (known as factor IX). Haemophilia B is also known as Christmas disease and is the cause of about 20% of cases.
Definition from NHS Choices: Factor IX and haemophilia B