F5 and deep vein thrombosis: Here, we report a multilocus genetic risk investigation in a large family carrying in cis a combined FV Leiden and SERPINC1 gene defect with high penetrance, early onset, and severe phenotypes, including severe DVT in two sisters during pregnancy characterized by a perinatal in utero unusual thrombotic event and a massive DVT, respectively, in spite of prophylactic antithrombotic therapy.