Within the past 20 years or so, factor IX has been at the centre of particularly intensive studies of its physiology, pathology and biochemistry as well as its molecular genetics and biology. With the complete nucleotide sequence of its human gene determined in 1985 and the molecular defects of over 600 abnormal human factor IX genes analysed to date, factor IX is among the few mammalian proteins which have been exhaustively studied in almost every aspect. The enormous amount of information we now have on this medium-sized plasma protein sheds light on how a gene and its protein evolve, how the protein carries out a highly regulated, specific and pivotal role in the delicately balanced blood coagulation reaction, and the correlation between clinical presentations and its highly diverse molecular mechanism of defects. This wealth of knowledge makes factor IX an excellent model for deeper study, such as truly quantitative analysis of its structure-function relationship andin vivofunction and regulation. It will also provide a sound foundation which may lead to improved treatment of haemophilia B and perhaps to its cure. This paper attempts to review the recent progress in research on factor IX.