Hemophilia is a rare genetic bleeding disorder in which a person inherits problems with certain blood clotting factors, making them unable to work properly. Blood-clotting factors are needed to help stop bleeding after a cut or injury and to prevent spontaneous bleeding.

Hemophilia genes can contain many different errors, leading to different degrees of abnormality in the amount of clotting factor produced. People who have hemophilia are at risk of abnormal bleeding throughout the body, especially in the joints and muscles, which may lead to disabling joint problems.