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 after a young boy named Stephen Christmas was found to be lacking this exact factor, leading to hemophilia, in 1952.

Physiology

Factor IX is inactive unless activated by factor XIa (of the contact pathway) or factor VIIa (of the tissue factor pathway). When activated into factor IXa, in the presence of Ca²⁺, membrane phospholipids, and a Factor VIII cofactor, it hydrolyses one arginine-isoleucine bond in factor X to form factor Xa.

Genetics

The gene for factor IX is located on the X chromosome (Xq27.1-q27.2). It was first cloned in 1982 by Kotoku Kurachi and Earl Davie.
Polly, a transgenic cloned Poll Dorset sheep carrying the gene for factor IX, was produced by Dr Ian Wilmut at the Roslin Institute in 1997.

Role in disease

Deficiency of factor IX causes Christmas disease (hemophilia B). Over 100 mutations of factor IX have been described; some cause no symptoms, but many lead to a significant bleeding disorder.

Some rare mutations of factor IX result in elevated clotting activity, and can result in clotting diseases, such as deep vein thrombosis.