To the Editor:Hyperhomocysteinemia (HHcy) is an important risk factor for atherothrombotic disease and venous thrombosis.[1] Normally,homocysteine (Hcy) is either remethylated to methionine (Met) by a Vitamin B12-dependent reaction or betaine as the methyl donor,or trans-sulfurated to cysteine through a pyridoxine-dependent reaction.Based on plasma level of total Hcy,HHcy is typically classified into three groups as follows:mild (tHcy 15-30 μmol/L),moderate (tHcy 30-100 μmol/L),and severe (tHcy ＞100 μmol/L).[2] Severe HHcy is rare and most commonly caused by inherited deficiency of cystathionine beta-synthase (CBS),or less frequently,ofmethylenetetrahydrofolate reductase (MTHFR) genes.The presentations of this metabolic disorder include developmental delay,intellectual disability,epilepsy,ectopia lentis,myopia,hypopigmentation of the skin and hair,marfanoid habitus,and thromboembolic disorders.[3] These manifestations are often overlooked in the early stage until the occur of thrombotic events like cerebral venous sinus thrombosis (CVST).