Over the past 15 years, our knowledge of the molecular basis of haemophilia B has increased dramatically. Following the cloning and characterization of the factor IX gene in 1982, major advances have been made in documenting the molecular pathology that underlies this condition. This review will summarize the current state of information in this area, and the reader is referred to the Haemophilia B Mutation Database World Wide Web site at http://www.umds.ac.uk/molgen/haemBdatabase for a complete current listing of the mutations that cause this phenotype. In addition, other recent reviews have discussed complementary issues relating to this topic.