Haemophilia B is due to a variety of mutations within the factor IX gene. In the Seattle series, 26 additional unrelated families have had a mutation identified within the past 2 years. Of these, 11 were common recurrent point mutations identifiable by rapid restriction digest screening; eight of these probably represent founder mutations. 15 others were identified by sequencing amplified coding region fragments; eight are novel. Two each had frameshift and donor splice mutations and 11 had missense mutations. Five of these mutations associated with normal levels of circulating dysfunctional factor IX were computer modelled into coordinates for factor IXa.