Partial gene deletion is the major cause of mutation leading to Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD). Partial gene duplication has also been recognized in a few cases. We have conducted a survey for duplication in 72 unrelated nondeletion patients, analyzed by Southern blot hybridization with clones representing the entire DMD cDNA. With careful quantitative analysis of hybridization band intensity, 10 cases were found to carry a duplication of part of the gene, a frequency of 14% for nondeletion cases (10/72), or 6% for all cases (10/181). The extent of these duplications has been characterized according to the published exon-containing HindIII fragment map, and in six of the 10 duplications a novel restriction fragment that spanned the duplication junction was detected. The resulting translational reading frame of mRNA has been predicted for nine duplications. A shift of the reading frame was predicted in four of the six DMD cases and in one of the two intermediate cases, while the reading frame remained uninterrupted in both BMD cases. RFLP and quantitative Southern blot analyses revealed a grandpaternal origin of duplication in four families and grandmaternal origin in one family. In all five families, the duplication was found to originate from a single X chromosome. Unequal sister-chromatid exchange is proposed to be the mechanism for the formation of these duplications.