Factor IX 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 after a young boy named Stephen Christmas was found to be lacking this exact factor, leading to hemophilia, in 1952. Factor IX is inactive unless activated by factor XIa or factor VIIa . When activated into factor IXa, it acts by hydrolysing...