In 1997, Simioni showed a cumulative recurrence rate of VTE of 39.7 percent among those with factor V Leiden mutation, with all recurrences occurring within three years, a rate 2.4 times higher than among individuals without the mutation. The factor V Leiden mutation is found in 4 to 6 percent of Caucasians and is the single most important cause of thromboembolism in a variety of conditions. Heterozygous carriers with the mutation have VTE at a younger age than do noncarriers. Among those with first VTE, the prevalence of the mutation is 15 to 40 percent and among those with a family history of VTE, as high as 50 percent. However, in a large study of men participating in the Physicians Health Study, those individuals with the mutation had an increased rate of VTE over time. These age-specific incidence rate differences ranged from 1.23 to 5.97 in those aged 70 or older. These data suggest that confounders other than genetic predisposition are important in the development of VTE.