Hemophilia B, or Christmas disease, a genetically inherited bleeding disorder characterized by inadequate amounts of clotting factor IX in blood plasma. Factor IX, a protein also known as plasma thromboplastin component (PTC), is involved in blood coagulation (clotting). Hemophilia B occurs in only about 70,000 men worldwide at any given time and occurs a fifth as frequently as does hemophilia A (factor VIII deficiency). Factor IX deficiency affects males only; females who inherit the condition are carriers. Carriers are usually, but not always, symptom- free. People with factor IX deficiency may need supplementary factor IX or fresh frozen plasma if they have a bleeding injury or are undergoing surgery.