Of the 50 women diagnosed with UVPT, 19 (38%) had raised D-dimer levels and 14 (28%) had abnormal initial thrombophilia screens: one mild protein C deficiency, one mild protein S deficiency, six were homozygous, and nine heterozygous for the C655T methylene tetrahydrofolate reductase (MTHFR) polymorphism. The gene discussed is MTHFR; the disease is hereditary thrombophilia due to congenital protein S deficiency.