Factor IX is also called Christmas factor or antihaemophilia B factor and is one of the essential components in blood coagulation. The protein is coded for by gene F9 and is found in an inactive state in the blood circulation. When a vessel is damaged, foreign bodies infiltrate. As soon as factor IX comes into contact with one of these it activates and enables the production of an enzyme, thrombokinase, which is then involved in the coagulation reactions leading to formation of a blood clot.
If this factor IX is absent, people suffer from haemophilia B.
Three-dimensional structure of factor IX. © Jawahar Swaminathan et EBI, Wikipedia, DP