Vitamin K- dependent protein S (ProS) is a plasma glycoprotein with anticoagulant properties [1]

Vitamin Kdependent protein S (ProS) is a plasma glycoprotein with anticoagulant properties [1]. Its anticoagulant function is well established in the literature based both in clinical data, genetics and laboratory assays. ProS is able to prolong the clotting time acting as cofactor for activated protein C. Furthermore, since the 80s it is known that hereditary ProS deficiency (PSD) increases the risk of venous thrombosis in families with hereditary thrombophilia [2], later found to be mostly caused by mutations in its coding gene, PROS1. Indeed, mutations in PROS1 have been shown to confer more than 7-fold risk of thrombosis in selected families with PSD [3]. The recent establishment of mice strains with mutations in Pros1 has emphasized the importance of ProS as a pleiotropic anticoagulant in vivo [4, 5].