Deadly Copies

FINDING CERTAIN GENES CAN BE AS DIFFICULT AS LOcating a phone number without an address or a last name. Ten years after researchers figured out that the gene that causes Huntington's disease, an uncommon degenerative brain disorder, was located somewhere on the fourth of 23 pairs of human chromosomes, an international team of scientists has pinpointed the actual hereditary entry responsible for the disease.

What they found was a biochemical equivalent of a debilitating stutter: a defective gene that includes too many copies of one crucial subsection of the DNA molecule. The more extra copies, the more severe the symptoms and the sooner they occur. In the past two years, three other conditions, including a form of muscular dystrophy, have been traced to such hereditary hiccups. With the Huntington's gene in hand, researchers can now develop more accurate diagnostic tests. They may also prove better able to predict at what age a person will succumb to the disease.