DE-NOVO EXPANSION OF A (CAG)(N) REPEAT IN SPORADIC HUNTINGTONS-DISEASE

Huntington's disease (HD) chromosomes contain an expanded unstable (CA G)n repeat in chromosome 4p16.3. We have examined nine families with p otential de novo expression of the disease. With one exception, all of the affected individuals had 42 or more repeat units, well above the normal range. In four families, elderly unaffected relatives inherited the same chromosome as that containing the expanded repeat in the pro band, but had repeat lengths of 34-38 units, spanning the gap between the normal and HD distributions. Thus, mutation to HD is usually assoc iated with an expansion from an already large repeat.