Recent discoveries of genes containing (CAG/CTG) repeats as the causative genes for neuromuscular diseases suggest that trinucleotide repeat expansions are associated with a number of other neurodegenerative diseases, particularly in those showing genetic anticipation. In order to efficiently identify expanded trinucleotide repeats, we have developed a novel method using a single-stranded probe containing (CAG)55, which allows direct detection of expanded (CAG/CTG) repeats in the myotonin-protein kinase (Mt-PK) genes of patients with myotonic dystrophy without the need for any prior information of chromosomal localizations of the disease genes. This method is expected to be useful for identifying novel genes for diseases associated with expanded (CAG/CTG) repeats.