Factor XI (FXI) is the zymogen of a serine protease enzyme in the intrinsic pathway of blood coagulation and is an important factor in the creation of a stable fibrin clot. Deficiency of FXI leads to an injury-related bleeding disorder and is remarkable for the lack of correlation between bleeding symptoms and FXI coagulant activity (FXI:C). The FXI protein is composed of five domains: four tandem repeat domains of approximately 80 residues known as Apple (Ap) domains, and the catalytic serine protease (Sp) domain. A total of 65 mutations throughout the FXI gene (F11) have been reported in FXI deficient patients. An interactive web database of these mutations has been created (www.FactorXI.org) that integrates the phenotypic data with genetic data and structural homology models for the five FXI domains. The database provides a central repository for all reported genetic alterations within F11. With the use of recently developed visualization tools, each mutation can be highlighted on the structural models of the FXI domains together with an appropriate survey of patient data, such as FXI:C levels and FXI antigen levels. The database also enables new F11 mutations to be interpreted. The interactive design of this database will lead to a more comprehensive comparative understanding of the genetic factors that influence bleeding risk