For the most part, modern medicine is no match for neurodegenerative diseases. But with advances in the study of genetics, the ability of scientists to get a molecular "handle" on such mysterious malfunctions promises to change all that. And perhaps the most useful such handle yet found is the phenomenon called trinucleotide repeat expansion.

This occurs when any three of the four nucleotide subunits in DNA material begin to excessively repeat their adjacent appearance in a molecule. For example, in normal nucleic acid molecules, the adjacent occurrence of cytosine, adenosine, and guanine (CAG) may repeat itself up to 40 times before being interrupted by another combination. But in recent years, researchers have observed CAG tracts of up to several hundred repeats in DNA of the victims of certain diseases. Since the CAG tract codes for glutamine, such ailments have become known as polyglutamine disorders.

At left, a slice from a...

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