Muscle dystrophin mRNAs from Duchenne (DMD) and Becker (BMD) patients with internal deletion of the DMD gene were quantitated and sequenced. In all cases (eight DMD and three BMD), truncated mature transcripts were found, and their amount was correlated to the clinical phenotype and to the reading frame. We focused on four cases that were apparently not in agreement with the reading frame rule. In two DMD cases, slightly reduced amounts of in-frame truncated mRNA are present but no dystrophin is detected, suggesting impaired translation and/or instability of the protein. In two BMD patients with out-of-frame deletions, the presence of minor in-frame alternatively spliced mRNA species is congruent with the observed truncated dystrophin and the mild phenotype.