Activated protein C (APC) resistance is the most frequent hereditary defect associated with deep vein thrombosis. This defect is caused by point mutation in the factor V gene resulting in a replacement of the amino acid Arg 506 by a Gln residue. The heterozygous defect is associated with a 5 to 10 fold, the homozygous defect with a 50 to 100 fold increased thrombosis risk.