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== People who have hemophilia may bleed for a long time after an injury or accident. They also may bleed into their knees, ankles and elbows. Bleeding in the joints causes pain and, if not treated, can lead to arthritis. Bleeding in the brain, a very serious complication of hemophilia, requires emergency treatment. The main treatment is injecting the missing clotting factor into the bloodstream.
 * Hemophilia** is a rare blood disorder that makes it so the blood does not clot normally. About 18,000 people in the U.S. have hemophilia. Each year, about 400 babies are born with the disorder. Hemophilia usually occurs in males.

Babylonian Jews first described hemophilia more than 1700 years ago; the disease first drew widespread public attention when Queen Victoria transmitted it to several European royal families. Mutation of the //HEMA// gene on the X chromosome causes Hemophilia A. Normally, females have two X chromosomes, whereas males have one X and one Y chromosome. Since males have only a single copy of any gene located on the X chromosome, they cannot offset damage to that gene with an additional copy as can females. Consequently, X-linked disorders such as Hemophilia A are far more common in males. The //HEMA// gene codes for Factor VIII, which is synthesized mainly in the liver, and is one of many factors involved in blood coagulation; its loss alone is enough to cause Hemophilia A even if all the other coagulation factors are still present.