If initial deletion/duplication analysis is negative, what is the next step in molecular testing for DMD?

• A. Repeat deletion/duplication analysis
• B. Sequencing of the DMD gene
• C. Muscle biopsy
• D. Karyotyping

Answer: (B)

When the initial screening for large deletions or duplications in the DMD gene comes back negative, the next step is sequencing the DMD gene. This is because many patients carry small-scale mutations—such as point mutations or small insertions/deletions—that do not change copy number and, therefore, aren’t detected by CNV analysis. Sequencing examines all coding exons and essential splice sites, and is typically done with next-generation sequencing (often with confirmation by another method if needed). If sequencing also finds nothing, further testing like RNA analysis to detect deep intronic or regulatory variants may be considered. Repeating the deletion/duplication test wouldn’t reveal small variants; muscle biopsy provides tissue pathology rather than mutation information; and karyotyping looks for whole-chromosome problems, not gene-level changes.