Today, there are many laboratories offering dystrophin-deletion testing. Most laboratories use multiplex PCR, and amplify 16 to 23 deletion-prone DMD exons. Since the deletion hotspot is from exon 44 to exon 53, more exons are amplified in this region. Although the 5' deletions are generally larger,it is important for laboratories to amplify about every third or fourth exon at the 5' end of the gene; otherwise, smaller deletions are missed. The multiplex strategies are a deletion test and will not detect duplications. Although Southern blot analysis is both costly and labor-intensive, duplications will be detected. Unlike the deletion distribution, duplications are found in about 5% to 8% of the patients and are located primarily at the 5' end of the gene. The identification of duplications is important both for the confirmation of the diagnosis and for accurate carrier studies for other family members. When laboratories encounter challenging nondeletion types of cases, Southern blot analysis should be performed. Proficiency testing for both DMD deletion testing and carrier testing is offered through the College of American Pathologists (CAP), and proficiency specimens are sent to participants twice per year.
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