Factor V Leiden thrombophilia is characterized by a poor clotting response leading to an increased risk of venous thromboembolism. The term "factor V Leiden" refers to the specific G-to-A substitution at nucleotide 1691 in the gene for factor V . Heterozygosity, carrying one copy of the mutation, slightly increases risk for bleeding. Homozygosity for the factor V Leiden allele and a much greater risk fro bleeding problems. Because it is relatively common, between 3-8% of Caucasian Americans are carriers, and treatment is not standardized, strategies for genetic testing and management are not clear.