<unk>  (or Christmas factor) () is one of the serine proteases of the coagulation system; it belongs to peptidase family S1. Deficiency of this protein causes haemophilia B. It was discovered in 1952 after a young boy named Stephen Christmas was found to be lacking this exact factor, leading to haemophilia. Coagulation<unk>  is on the World Health Organization's List of Essential Medicines.
Factor IX
Factor VIII
Factor VIII 
Factor XI
Fibrinogen
Thrombin
Prothrombinase
Β-Thromboglobulin