Approximately half of the transcripts of the DMD gene isolated from peripheral blood lymphocytes and detected by RT-PCR do not contain exon 9. This 'exon skipping' can be observed to a variable extent in all tissues not specifically expressing the gene product 'dystrophin'. The shorter transcript is rare in muscle, heart and brain. Similar results were found in mice. Since the 3' end of exon 9 reflects the consensus sequence of a 3' splice site, it is suggested that exon 9 due to this sequence element is 'recognized' as an intron and removed from an RNA intermediate. This model is supported by a mathematical comparison of real and putative splice sites within the gene.