The most frequent causes of an inherited (primary) hypercoagulability state are factor V Leiden mutations and prothrombin gene mutations, which together account for 50% to 60% of the cases.2–6 Defects in protein S, protein C, and antithrombin (formerly known as antithrombin III) account for most of the remaining cases, while dysfibrinogenemia is a rare cause.10–12 In the present case, no protein C or factor V mutations were found, based on which hereditary thrombophilia was excluded. The gene discussed is PROS1; the disease is familial hypodysfibrinogenemia.