Factor IX Deficiency (F9)
Factor IX deficiency, also known as Hemophilia B, is an X-linked, pan-ethnic bleeding disorder that is caused by deficiency of clotting factor IX. Factor IX is encoded by the gene F9. Due to X-linked inheritance, most patients are males, although approximately 10% of female carriers may have symptoms. Clinical features may be severe, moderate or mild, with the age of diagnosis earliest for severe forms. In severe hemophilia B, patients bleed spontaneously and heavily several times a month; in the moderate form, patients continue to bleed for longer than expected after injury; and in the mild form, patients may not be diagnosed until adults, where they may have excessive bleeding after surgeries. Null variants tend to cause the severe form of the disease, whereas missense variants may be associated with any of the forms. Without treatment, life expectancy for patients with the severe form is in childhood, but with modern treatments, life expectancy is now greater than 60 years old.
For information about carrier frequency and residual risk, please see the residual risk table.