The enzyme vitamin K 2, 3 epoxide reductase (VKOR) is the target for anticoagulants. In a biochemical process in the liver cells, the so called vitamin K cycle, this enzyme enables each vitamin K molecule ingested in food to be recycled about 10,000 times. Vitamin K in its reduced form (vitamin K hydroquinone) is an essential co-factor for the carboxylation of glutamate residues to produce calcium-binding, gamma-carboxyglutamate residues (Gla). This post-translation step is required for the activation of precursor proteins in the production of the active blood clotting factors II, VII, IX and X. Similar vitamin K-dependent Gla-proteins are also known to play key roles in the regulation of a number of other proteins, including one involved with bone metabolism. Having a similar structure in the binding site as the vitamin K molecule, anticoagulants may block the VKOR enzyme. If the reduction step is inhibited, the recycling process stops, leading to impairment of blood coagulation and spontaneous haemorrhages as soon as the supply of vitamin K hydroquinone is depleted. Modifications in the protein structure due to polymorphisms on the gene coding the VKOR may induce anticoagulant resistance.