Abstract: The present invention includes a DNA clone from the myotonic muscular dystrophy gene, a cosmid probe to the myotonic dystrophy site, as well as methods of detecting myotonic muscular dystrophy using RFLP. The method involves the steps of digesting DNA from an individual to be tested with a restriction endonuclease and detecting the restriction fragment length polymorphism with hybridization to probes within the myotonic muscular locus and southern blot analysis. Alternatively, the myotonic muscular dystrophy gene can be measured by determining the amount of mRNA or measuring the amount of protein with an antibody. Further, the myotonic muscular dystrophy gene defect can be detected using either fluorescence in situ hybridization or pulsed field gel electrophoresis using the probes described herein.