Case Report The association between hyperhomocysteinemia and venous thrombosis, which has been controversial for a long time, is now widely confirmed. The causes of hyperhomocysteinemia are multiple, both hereditary and acquired. The most common hereditary abnormality is the C677T mutation of methylenetetrahydrofolate reductase (MTHFR) in a heterozygote state. Hyperhomocysteinemia is a risk factor for venous thrombosis that should be investigated as part of the Thrombophilia assessment. The

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... rvation of a 21 year old patient with hyperhomocysteinemia diagnosed by a thrombosis of the subclavian vein will be reported first, then the role of this abnormality in the occurrence of arterial and venous thrombotic incidents will be recalled.