The American Heritage® Dictionary of the English Language, 4th Edition
n. An amino acid used normally by the body in cellular metabolism and the manufacture of proteins. Elevated concentrations in the blood are thought to increase the risk for heart disease by damaging the lining of blood vessels and enhancing blood clotting.
n. An amino acid which is monitored in the blood to estimate risk of cardiovascular disease.
Word Usage
"The breakthrough centres on a compound called homocysteine which is naturally made in the body and, at high levels, has been linked to memory loss and Alzheimer's."