WordNet
factor IX
n. coagulation factor whose absence is associated with hemophilia B [syn: Christmas factor]
Wikipedia
Factor IX
Factor IX (or Christmas factor) is one of the serine proteases of the coagulation system; it belongs to peptidase family S1. Deficiency of this protein causes hemophilia B. It was discovered in 1952 after a young boy named Stephen Christmas was found to be lacking this exact factor, leading to hemophilia.
It is on the WHO Model List of Essential Medicines, the most important medications needed in a basic health system.