Hemophilia B, also known as factor IX (FIX) deficiency or Christmas disease, is a genetic disorder caused by missing or defective factor IX, a clotting protein. It is a rare inherited bleeding disorder that affects mostly males, but carrier females may also show some signs of bleeding. Hemophilia B is caused by an inherited X-linked recessive trait, with the defective gene located on the X chromosome. Females have two copies of the X chromosome, so if the factor IX gene on one chromosome does not work, the gene on the other chromosome can do the job of making enough factor IX. All female children of men with hemophilia carry the defective gene.

Symptoms of hemophilia B can vary in severity, with prolonged bleeding being the main symptom. It is often first seen when the infant is circumcised, and other bleeding problems usually show up when the infant starts crawling and walking. Severity of symptoms can be categorized according to the degree of factor IX deficiency in the blood:

• Mild hemophilia B: 6% up to 49% of FIX in the blood. People with mild hemophilia B typically experience bleeding only after serious injury, trauma, or surgery. In many cases, mild hemophilia is not diagnosed until an injury, surgery, or tooth extraction results in prolonged bleeding. The first episode may not occur until adulthood. Women with mild hemophilia often experience menorrhagia, heavy menstrual periods, and can hemorrhage after childbirth.
• Moderate hemophilia B: 1% up to 5% of FIX in the blood. People with moderate hemophilia B tend to have bleeding episodes after injuries. Bleeds that occur without obvious cause are called spontaneous bleeding episodes.
• Severe hemophilia B: <1% of FIX in the blood. People with severe hemophilia B experience bleeding following an injury and may have frequent spontaneous bleeding episodes, often into their joints and muscles.

The main medication to treat hemophilia B is concentrated FIX product, called clotting factor or simply factor. Recombinant factor products, which are developed in a lab through the use of DNA technology, preclude the use of human-derived pools of donor-sourced plasma. Plasma-derived FIX products are still available, but approximately 75% of the hemophilia community takes a recombinant product. Living with hemophilia B means being vigilant about receiving treatment and taking extra steps to protect overall health.