Haemophilia is a rare and serious X-chromosome linked congenital bleeding disorder that affects the blood's ability to clot, meaning that people with haemophilia bleed for a longer time than normal. It is estimated that about 1 in 10,000 people are affected by haemophilia, with 450,000 people living with haemophilia worldwide.
Haemophilia is characterised by deficiencies of coagulation factors, and is typically passed down from parent to child, although about a third of cases are caused by a spontaneous mutation. There are two different types of haemophilia, each associated with deficiency of a particular coagulation factor.
The most common type is haemophilia A, where the person does not have enough coagulation factor VIII (FVIII).
Haemophilia B is less common, with people not having enough coagulation factor IX (FIX), representing only 15–20% of total haemophilia cases.
The genes for both coagulation factors are on the X chromosome, which is why it mainly affects males who inherit an affected maternal X chromosome.