The 20210 G to A variation in the prothrombin gene was first described in 1996 and found to be associated with both elevated plasma prothrombin levels and a moderately increased risk for first venous thrombotic events. These findings generally have been confirmed in several independent studies. The 20210A allele seems to be rather common in white subpopulations, among whom the prevalence of heterozygous carriers may vary from 1% to 5%. The variant prothrombin allele contributes to the risk of thrombosis in carriers of other genetic risk factors (eg, factor V Leiden); also, this allele seems not to be an important risk factor for arterial thrombosis, unless risk factors for cardiovascular disease (eg, smoking) also are present.