Amniocentesis: a prenatal test performed during the second trimester of pregnancy to check for abnormalities in the fetus.

Ashkenazi: Jews whose ancestors emigrated from eastern and central Europe, particularly Germany, Poland and Russia. More than 90 percent of Jews in the United States are of Ashkenazi descent.

Aspartoacyclase (ASPA): a brain enzyme that is deficient in patients with Canavan disease and ultimately leads to the spongy condition of the brain’s "white matter," resulting in the disease’s incapacitating symptoms.

Carriers: individuals whose genetic makeup carries the potential to pass on to their children a specific mutated gene that can lead to a disease (such as Canavan), even though these carriers are not themselves at risk for the disease.

Chorionic villus sampling (CVS): a prenatal test performed during the first trimester of pregnancy to check for abnormalities in the fetus.

DNA (deoxyribonucleic acid) analysis: in testing for Canavan disease and other genetic disorders, a blood sample is analyzed to detect any genetic abnormalities found in the DNA, a nucleic acid that contains the genetic code and transmits the hereditary pattern.

Leukodystrophies: a group of inherited neurological disorders (such as Canavan) that affect the growth of the myelin sheath.

Myelin sheath: the "white matter" of the brain that serves as an insulator to protect nerves. In Canavan disease, a chemical imbalance destroys the myelin and causes the "white matter" to become spongy, resulting in the disease’s incapacitating symptoms

N-acetylaspartic acid (NAA): a deficiency of the aspartoacylase enzyme causes this substance (NAA) to accumulate in the brain, resulting in a chemical imbalance that destroys the myelin sheath.

Recessive trait: an inherited trait (such as Canavan disease) that requires both parents (carriers) to pass on copies of the abnormal gene to their child. (If only one gene is necessary to cause a disease--unlike in Canavan disease--the trait is said to be dominant.)