A mutation G ® A in the untranslated 3? region of the prothrombin gene at nucleotide position 20210 (G20210A mutation) constitutes a risk factor for venous thromboembolism. About 90% of carriers of the mutation have elevated levels of prothrombin activity. The normal range for non-carriers is about 75-130%. There is a research method available to measure prothrombin activity levels up to 175% using the thrombin substrate S-2238. It is not specific for the G20210A mutation.