Hereditary protein S deficiency.

Protein S is a vitamin K-dependent plasma protein that serves as a cofactor of activated protein C(APC) in its inhibitory action on activated factor V and factor VIII and in its stimulation of fibrinolytic activity. In plasma, part of the protein S is complexed with the C4b-binding protein. Only the free protein S has APC cofactor activity. In our laboratory, 30 patients from 8 nonrelated families were detected that fulfilled the criteria of an isolated protein S deficiency. All patients were heterozygotes for the defect that is inherited as an autosomal-dominant disorder. Patients with a protein S deficiency were found to be at risk for the development of venous thrombotic disease at a relatively young age.