Duchenne muscular dystrophy (DMD) is the most common of the muscular dystrophies affecting one in 3,000 live male births. Both DMD and the mild form, Becker muscular dystrophy (BMD), are X-linked. There are a number of females affected by the disease who all possess an X-autosome translocation, with the exchange point in the X always occurring within chromosome band Xp21. This, together with linkage and deletion data, has localized the gene at band Xp21. DNA fragments from this region have been cloned using a patient with a large Xp21 deletion and from a patient with a t(X:21) translocation. The former clones (pERT 87) comprise the DXS164 locus and the latter clones (XJ) the DXS206 locus. Subclones from both regions allow the detection of deletions in approximately 11% of DMD patients. A fetal muscle complementary DNA clone corresponding to exons in the DXS164 locus has been isolated and detects a 16-kilobase (kb) transcript. We present the isolation of an adult muscle cDNA clone from the DXS206 locus that detects a 16-kb mRNA in adult human muscle. The cDNA clone contains exons that map in the DXS206 locus, the DXS164 locus, and on the centromeric side of these cloned regions. The t(X;21) translocation exchange points occurs within a large intron of 105 kb or larger, indicating that the translocation has disrupted the DMD/BMD gene to cause the disease in this patient.