Hemophilia (or haemophilia) B, a blood-clotting disorder in which a mutation of the Factor IX gene leads to a deficiency of Factor IX (or Christmas factor), a serine protease of the coagulation system. Both the factor and the disease are named for Stephen Christmas (not the holiday), the first patient discovered to have the condition in 1952. We haven't let our daughter participate in any more physical sports since she was diagnosed with Christmas disease last year.