We describe a patient with a duplication of more than 400,000 bp of the dystrophin gene. The duplication is completely contained within the gene, and the duplicated exons are predicted to be "in frame" with the rest of the gene. Dystrophin protein is detected in the patient's muscle as a single species of approximately 600 kDa (normal, approximately 400 kDa), indicating that the resulting mutated gene codes for a translatable mRNA of over 100 exons (normal, approximately 70 exons). The patient's mother carries the duplicated gene as determined by both DNA and protein analysis. The described duplication of the dystrophin gene is by far the largest characterized to date. This observation is of significant biologic interest in that, despite the gross alteration of the gene and the encoded protein, the patient has a relatively mild clinical progression compatible with a diagnosis of Becker muscular dystrophy.