Factor V Leiden and the Prothrombin 20210A Gene Variant After first-degree relatives had completed assessments for previous VTE, their index cases will categorized as positive for factor V Leiden or the prothrombin 20210A gene variant, or negative for both. Personnel who will be unaware of the propositus family history of VTE, or the subject's past history of VTE, will perfome these assays in a central laboratory in France.