Using the direct identification of repeat expansion and cloning technique, we cloned a novel long CAG/CTG trinucleotide repeat on chromosome 17. Using radiation hybrid panels, the CAG/CTG repeat was mapped to chromosome 17q. The CAG/CTG repeat is highly polymorphic, with a heterozygosity of 85%, and exhibits a bimodal distribution (allele S, 10-26 repeat units, and allele L, 50-92 repeat units). The CAG/CTG repeat of allele L exhibited intergenerational instabilities, which are more prominent in maternal transmission than in paternal transmission. Analyses of Northern blot and RT-PCR indicate that the repeat is transcribed. Although the size of the CAG/CTG repeat of allele L is within the range of the expanded CAG repeat of disease-causing genes, we did not detect any association of allele L with various neurodegenerative diseases, including frontotemporal dementia and parkinsonism, mapped to 17q21-q23.