Although family and twin pair studies provide evidence of the contribution of genetic components to arterial thrombotic disease, these are difficult to detect because of their high number, small individual effect on thrombosis, and complex interaction with environmental factors. Despite extensive investigation of several candidate mutations, particularly in genes involved in the hemostatic process, there is still only weak reproducible evidence for their role in arterial thrombosis. In contrast, a number of defects and functional polymorphisms lead to increased risk for venous thromboembolism. Taken together, epidemiological and molecular studies suggest that we need to study multiple DNA-sequence variations defining functional haplotypes and their relationships with environmental factors in large cohorts of patients with better-defined end point phenotypes...