From the human genome sequence, select ? enzymes that will bracket the region of the huntingtin gene that contains the triplet repeat. Obtain DNA from members of families afflicted with Huntington's disease. Carry out Southern blotting with the selected ? enzymes. Probe each blot with an oligonucleotide ? or its complement ? . Or PCR amplify the region of interest and run gel electrophoresis to estimate, from the size of the repeat fragment, the number of repeats; this is what is actually done in HD testing. The age of onset should correlate ? with the size of the gel band detected by the probe; the larger the band, the ? the age of onset.