Hyperhomocysteinemia refers to above-normal concentrations of plasma/serum homocysteine. Plasma/serum homocysteine is the sum of the thiol-containing amino acid homocysteine and the homocysteinyl moiety of the disulfides homocystine and cysteine-homocysteine, whether free or bound to proteins. Hyperhomocysteinemia in isolation may be associated with an increased risk of atherosclerosis and recurrent arterial and venous thrombosis usually in the third or fourth decade of life. [From OMIM]