In dystrophin Kobe exon 19 of the dystrophin gene is skipped during the process of mRNA precursor splicing even though the splice sites are unchanged (Matsuo et al. J. Clin. Invest. 87:2127-2131,1991). In the predicted secondary structure of the mRNA precursor, exon 19 of dystrophin Kobe is paired with intron sequences, whereas a large part of exon sequence from wild type is paired with itself and folded into a large hairpin structure. As all of 22 additional dystrophin exons analyzed also form intra-exon hairpin structures, these structures may be considered essential components of exons. We suggest that the abolishment of a hairpin structure in the truncated exon of dystrophin Kobe might prevent the splicing machinery from recognizing the splice sites and induce exon skipping.