Factor X (FX) deficiency is a rare, recessively inherited bleeding disorder representing 10% of all rare bleeding diseases and affecting 1 in every 1,000,000 people. Its clinical presentation places FX deficiency among the most severe of the rare coagulation defects, typically including hemarthroses, hematomas, and umbilical cord, gastrointestinal, and central nervous system bleeding. Phenotype diagnosis is based on the concomitant prolongation of the prothrombin time and activated partial thromboplastin time. Through the measurement of plasma level of FX antigen and its coagulant activity, two main types of deficiency can be distinguished: type I (concomitantly low levels of activity and antigen) and type II (low coagulant activity, but normal or borderline antigen levels). FX protein is mainly synthesized by the liver and is encoded by a gene (F10) of 27 kb located on chromosome 13, containing 8 exons. One hundred five mutations on F10 have been identified to date, 78% being missense mutations, with no hot-spot regions. There is no specific FX concentrate available, and current treatment includes the administration of fresh-frozen plasma or prothrombin complex concentrates (PCCs) containing FX in addition to other vitamin K–dependent factors. Administration of PCCs is associated with the risk of thromboembolic complication due to the unknown concentrations of other coagulant factors; however, to overcome this problem, a concentrate containing well-defined amounts of FX (and FIX) has recently been developed.