Factor IX (or Christmas factor or Christmas-Eve factor) is one of the serine proteases () of the coagulation system; it belongs to peptidase family S1. Deficiency of this protein causes hemophilia B.

Physiology

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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, it acts by hydrolysing one arginine-isoleucine bond in factor X to form factor Xa. It requires calcium, membrane phospholipids, and factor VIII as cofactors to do so.

Genetics

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The gene for factor IX is located on the X chromosome (Xq27.1-q27.2).

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