Homocysteine is an amino acid that is produced by the body by chemically altering adenosine. Amino acids are naturally made products, which are the building blocks of all the proteins in the body. In 1969, Dr. Kilmer S. McCully reported that children born with a genetic disorder called homocystinuria, which causes the homocysteine levels to be very high, sometimes died at a very young age with advanced atherosclerosis in their arteries. Homocysteine levels in the blood may be elevated for many reasons. More specifically, these reasons can be divided into severe genetic causes and other milder causes.