Factor IX

Editors-In-Chief: John Alexander, M.D. [mailto:john.h.alexander@duke.edu], Duke Clinical Research Institute; C. Michael Gibson, M.S., M.D. Duke Clinical Research Institute [mailto:mgibson@perfuse.org]

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, 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
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.

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.