X, factor: A coagulation factor, a substance in blood essential to the normal clotting process. Production of factor X takes place in the liver and requires vitamin K. The gene for factor X is located on chromosome 13 and is in band 13q34.

The "X" in factor X is the Roman numeral "ten." (All numbered coagulation factors bear Roman numerals.) Factor X became known because of a genetic condition in which the factor is lacking.

Factor X deficiency is inherited as an autosomal recessive trait manifest by prolonged nose bleeds, gastrointestinal hemorrhage, menorrhagia (abnormally heavy menstrual bleeding), hematuria (blood in the urine), and hemarthrosis (bleeding into joints). Pregnancy in women with factor X deficiency is often associated with adverse fetal outcomes (recurrent spontaneous abortion, placental abruption, and premature birth).

Factor X is also called Stuart-Prower factor because Mr. Stuart and Miss Prower were the first persons shown to have deficiency of this factor.