The analysis of DNA from patients suffering from Duchenne (DMD) and Becker (BMD) muscular distrophies has resulted in the identification of a single gene locus for these diseases. The locus is deleted to varying extents in affected patients. The translation product of this locus has been implicated as the site of the primary biochemical defect responsible for these muscle disorders. There is no simple correlation between the severity of the clinical phenotype and the location and extent of genomic deletions in the DMD locus. This lack of correlation may be due, in part, to the difficulties inherent in examining a gene of complex arrangement, with exons distributed over a large genomic distance. This paper examines the location of deletion breakpoints in DMD and BMD patients. The molecular analysis of these deletions are presented in the context of transcriptional and translational studies of DMD gene expression and the manifestation of the clinical phenotype.