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The Huntington's disease (HD) gene has been mapped in 4~16.3 but has eluded identification. We have used haplotype analysis of linkage disequilibrium to spotlight a small segment of 4~16.3 as the likely location of the defect. A new gene, IT15, isolated using cloned trapped exons from the target area contains a polymorphic trinucleotide repeat that is expanded and unstable on HD chromosomes. A (CAG), repeat longer than the normal range was observed on HD chromosomes from all 75 disease families examined, comprising a variety of ethnic backgrounds and 4~16.3 haplotypes. The GAG), repeat appears to be located within the coding sequence of a predicted-346 kd protein that is widely expressed but unrelated to any known gene. Thus, the HD mutation involves an unstable DNA segment, similar to those described in fragile X syndrome, spino-bulbar muscular atrophy, and myotonic dystrophy, acting in the context of a novel 4~16.3 gene to produce a dominant phenotype.

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