Issues in Huntington's disease testing.

The gene that is mutated in Huntington's disease has a polymorphic (CAG)n tract close to the 5' end of its message that is unstable and abnormally expanded in disease chromosomes. Rapid PCR tests that measure the CAG repeat number in this gene will become an important clinical tool. However, this test should be approached with caution, as its specificity and sensitivity have not been absolutely determined. The possibility that one might be able to predict the age of onset from the size of the CAG expansion in cases that have more than 50 repeats has been considered. We have also made a case for retaining the diagnostic option of prenatal exclusion testing.