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Protein Z is a vitamin K-dependent protein. The first cases of protein Z deficiency were described in patients suffering from a bleeding tendency from otherwise unknown origin. Today, diminutions of protein Z seem to play a role not only in bleeding patients but also in patients with factor V Leiden mutation: Patients presenting with factor V Leiden mutation and low protein Z levels show earlier onset and higher frequency of thromboembolic events than do patients presenting with factor V Leiden mutation and normal protein Z levels. Thus, protein Z is a good example for the—at first sight—paradox action of coagulation proteins.