Myotonic dystrophy (MD), occurring in about 1 of 8000 individuals, is the most common form of muscular dystrophy in adults. The disease, which is characterized by progressive muscle degeneration, is caused by a dominant mutant gene that contains an expanded CAG repeat region. Wild-type alleles of the MD gene contain 5–30 copies of the trinucleotide. Mutant MD alleles contain 50 to over 2000 copies of the CAG repeat. The complete nucleotide sequence of the MD gene is available. Design a diagnostic test for the mutant gene responsible for myotonic dystrophy that can be carried out using genomic DNA from newborns, fetal cells obtained by amniocentesis, and single cells from eight-cell preembryos produced by in vitro fertilization.