Most Duchenne muscular dystrophy (DMD) patients have genetic deletions or point mutations in the dystrophin gene that alter the reading frame of dystrophin mRNA. This causes early termination of translation, and no dystrophin (or, less commonly, a truncated N-terminal dystrophin fragment) is produced. In many DMD patients, however, a small proportion of muscle fibers show strong dystrophin staining, and these" revertant fibers" are thought to arise by a mechanism that restores the reading frame. Exon-specific monoclonal antibodies (mAbs) have now been used to determine, for the first time, which exons are removed, in order to correct the reading frame in individual muscle fibers. Thus, 15 revertant fibers in a DMD patient with a frameshift deletion of exon 45 were shown to correct the frameshift by the additional deletion of exon 44 (or perhaps exon 46 in some fibers) from the dystrophin mRNA, but not by larger deletions. This result was consistent with reverse transcription (RT)-PCR and sequencing of a minor dystrophin mRNA with an exon 43/46 junction in this biopsy. In a DMD patient with a frameshift deletion of exons 42 and 43, however, larger deletions than the minimum necessary were used to correct the frameshift. In this patient, who produces a half-size N-terminal dystrophin fragment in all fibers, we were able to show that the revertant dystrophin replaces the truncated dystrophin in revertant-fiber sarcolemma. The results are consistent with somatic mutations in revertant-fiber nuclei, which result in removal of additional exons from dystrophin mRNA.(ABSTRACT TRUNCATED AT 250 WORDS)