The myotonic dystrophy (DM) expansion varies from 50 to 4000 CTG repeats in the 3' untranslated region of the DMPK gene. Direct analysis by Southern blot, after restriction enzyme digestion of genomic DNA, is the method of choice for studying the DM mutation. A long polymerase chain reaction (PCR)-formatted protocol, which involved a single genomic in vitro amplification followed by high concentration agarose gel electrophoresis and oligo-specific hybridization, was used to amplify normal alleles and DM alleles in all examined ranges of expansion (up to 3,700 CTGs) starting from a small amount of genomic DNA (> or = 15 pg). This method is quick, sensitive, and reproducible and reduces the cost of diagnostic laboratory processing.