A thrombophilia work-up (lupus anticoagulant, anticardiolipin and anti-β2-glycoprotein I antibodies, factor II G20210A mutation, factor V Leiden mutation, anti-thrombin, protein C and protein S) showed low level of free protein S antigen (37% normal > 55%). The gene discussed is PROS1; the disease is Rare hereditary thrombophilia.