In slightly over a quarter of all Duchenne patients (28%) a small mutation is found in the dystrophin gene. For these patients all exons are present, but a single DNA subunit (and thus of the RNA copy) is changed (figure 7). This change causes a premature stop signal and the protein translation stops too soon.
In addition to point mutations (one DNA subunit is replaced by another) it can also occur that one or two DNA subunits disappear or are added in an exon. This disrupts the genetic code in a similar way as when deletions and duplications happen.
Finally, small mutations can disrupt the splicing process (where introns are removed and exons are joined to form the genetic code). The start and end of all introns contain the same, specific combination of subunits, which are recognized by the splicing machinery (figure 2). If one of these specific subunits is changed, an exon will no longer be recognized, which can disrupt the genetic code (figure 3 and 4).