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Haemophilia B results from a congenital deficiency of Factor IX coagulant activity. It is an X-linked recessive disorder affecting 1 in 30 000 males.
It 'breeds' true so that family members tend to be all severely affected or all mildly affected.
It is clinically indistinguishable from haemophilia A, presenting with recurrent haemorrhage, spontaneously or post-operatively, into soft tissues and joints.