The DMD gene contains 79 exons, and which ones are disrupted, deleted, or skipped determines whether the body produces functional dystrophin, a truncated but partly working version, or essentially none at all. This distinction is the molecular boundary between Duchenne muscular dystrophy, which is severe and progressive, and Becker muscular dystrophy, which is milder and more variable. The relationship between individual exons and protein output is not as simple as “missing piece, broken machine,” though. Some exons encode dispensable stretches of the protein, while others encode regions so critical that even small disruptions there abolish function entirely.
The Largest Human Gene and Its 79 Exons
The DMD gene spans roughly 2.2 million base pairs of DNA, making it the longest gene in the human genome. Despite that enormous size, the actual protein-coding sequences account for only about 0.7% of the gene. The rest consists of introns, the non-coding stretches between exons, which average around 28,000 base pairs each and range from just 107 base pairs to a staggering 360,000 base pairs. Transcribing the full gene takes approximately 16 hours.1Taylor & Francis Online (RNA Biology). Non-sequential and multi-step splicing of the dystrophin transcript The gene also uses seven different promoters and two polyadenylation sites, generating multiple transcript variants beyond the full-length muscle form.
This architecture matters because the sheer length and complexity of the gene make it unusually vulnerable to large-scale mutations. Deletions of one or more exons are by far the most common type of mutation in patients, and the consequences depend heavily on which exons are lost and whether the remaining exons can still be stitched together into a readable protein blueprint.
The Reading Frame Rule
In 1988, researchers proposed a framework that still guides thinking about DMD mutations: the reading frame rule. The idea is straightforward. If a deletion or duplication shifts the reading frame of the mRNA so that the ribosome can no longer read the remaining code correctly, the result is a premature stop signal. The cell produces a short, unstable, nonfunctional scrap of protein, and the patient develops Duchenne muscular dystrophy. If the mutation removes exons but leaves the reading frame intact, the ribosome can still translate the remaining message into a shorter-than-normal but partially functional dystrophin. That patient typically develops the milder Becker muscular dystrophy.2PubMed. Entries in the Leiden Duchenne muscular dystrophy mutation database: an overview of mutation types and paradoxical cases that confirm the reading-frame rule
The distinction hinges on exon boundaries. Each exon contributes a certain number of nucleotides to the mRNA. If the exons flanking a deletion happen to line up so that the remaining code stays in phase, the protein will be internally shortened but readable. If they don’t line up, the whole downstream message becomes garbled. This is why two patients can both be missing exons in the same general region of the gene yet have drastically different clinical outcomes. One deletion preserves the reading frame; the other does not.
The Deletion Hotspot Between Exons 45 and 53
Deletions in the DMD gene do not scatter randomly across all 79 exons. The most frequent cluster falls in the region between exons 45 and 53, a recognized mutational hotspot.3PubMed Central. A duchenne muscular dystrophy gene hot spot mutation in dystrophin-deficient cavalier king charles spaniels is amenable to exon 51 skipping A second, smaller hotspot exists near the 5′ end of the gene, around exons 2 through 20. The clustering in these regions likely reflects structural features of the DNA that make certain stretches more prone to breakage and recombination during cell division.
For treatment, the hotspot has an important practical consequence: therapies designed to address mutations in this region can, in theory, help a disproportionately large share of patients. Exon 51 skipping alone is predicted to benefit more patients than any other single-exon strategy, precisely because so many common deletions land in or near this zone.
Not All Exons Are Created Equal
Dystrophin is a large structural protein that acts as a shock absorber, connecting the internal skeleton of a muscle fiber to the proteins embedded in the cell membrane. Different regions of the protein handle different jobs, and the exons encoding those regions vary dramatically in how much damage their loss inflicts.
The central portion of dystrophin consists of a long series of spectrin-like repeats, rod-shaped segments that were once considered largely redundant. Lose a few of these repeats, and the protein still works reasonably well, which is why many Becker patients with in-frame deletions in this region retain substantial muscle function. But research in mouse models has shown that these repeats are not as interchangeable as once assumed. They harbor binding sites for other proteins, and some repeats contribute more to function than others.4Biochimica et Biophysica Acta (BBA) – Proteins and Proteomics. Dystrophin: more than just the sum of its parts For example, inducing an in-frame deletion of exons 56 and 57 in mice produced a more functional dystrophin than deleting exons 58 and 59, even though both deletions removed a similar amount of the rod domain. The exon 56–57 deletion led to higher dystrophin expression and better stabilization of proteins at the muscle membrane.5PubMed Central. Morpholino Oligomer-Induced Dystrophin Isoforms to Map the Functional Domains in the Dystrophin Protein
The cysteine-rich domain near the tail end of dystrophin is a different story. This region is where dystrophin physically grips the dystrophin-associated glycoprotein complex at the cell membrane, the molecular handshake that keeps everything anchored. Lose this domain and the protein may still travel to the membrane, but it cannot do its job. An analysis of a patient whose deletion precisely excised the cysteine-rich and adjacent C-terminal domains found the dystrophin sitting at the membrane yet functionally useless, resulting in severe Duchenne-type disease.6JCI Insight. An intact cysteine-rich domain is required for dystrophin function Binding studies confirmed that the glycoprotein-binding site is confined to this cysteine-rich domain and the first half of the C-terminal domain, matching the clinical observation that deletions here cause the most severe outcomes.7PubMed. Glycoprotein-binding site of dystrophin is confined to the cysteine-rich domain and the first half of the carboxy-terminal domain
In practical terms, a therapy that produces a shortened dystrophin missing some central rod repeats has a far better chance of being functional than one that disrupts the cysteine-rich domain. This is the biological rationale behind many exon-skipping strategies: they aim to remove exons from the middle of the gene while preserving both ends.
Exon Skipping as a Therapeutic Strategy
The logic of exon skipping is to work with the reading frame rule rather than against it. If a patient’s deletion has thrown the reading frame off, you can use a short synthetic molecule called an antisense oligonucleotide (AON) to hide an additional exon from the cell’s splicing machinery. The spliceosome, the cellular complex that stitches exons together, simply does not see the targeted exon and leaves it out along with the flanking introns. If the right exon is removed, the reading frame snaps back into alignment, and the cell produces a truncated but partly functional dystrophin instead of none at all.8PubMed Central. Antisense Oligonucleotide-Mediated Exon-skipping Therapies: Precision Medicine Spreading from Duchenne Muscular Dystrophy
The AON works by binding to its target site on the pre-mRNA during transcription, physically blocking splicing factors from recognizing that stretch as an exon.9PLOS ONE. Dynamics of Co-Transcriptional Pre-mRNA Folding Influences the Induction of Dystrophin Exon Skipping by Antisense Oligonucleotides The approach is versatile: different AONs can be designed to skip different exons, and mixing AONs can target multiple exons at once.10PubMed Central. Antisense-mediated exon skipping: a versatile tool with therapeutic and research applications
Approved Exon-Skipping Drugs
Eteplirsen (marketed as Exondys 51) was the first exon-skipping drug to receive accelerated approval from the U.S. FDA, in September 2016. It targets exon 51, inducing its removal during splicing to restore the reading frame for patients whose deletions are amenable to that correction.11PubMed Central. Eteplirsen in the treatment of Duchenne muscular dystrophy Early proof-of-concept work had shown that a morpholino oligonucleotide injected into muscle could skip exon 51, restore the open reading frame, and lead to detectable dystrophin expression along with reassembly of the dystrophin-associated glycoprotein complex.12PubMed Central. Restoration of the dystrophin-associated glycoprotein complex after exon skipping therapy in Duchenne muscular dystrophy
A second drug, viltolarsen, targets exon 53. Exon 53 skipping can theoretically treat about 8% of all patients with Duchenne, including those with deletions of exons 45–52, 47–52, 48–52, 49–52, 50–52, or exon 52 alone, among others. The drug binds to exon 53 on the pre-mRNA, hides it from the splicing machinery, and the resulting in-frame transcript is translated into a truncated but functional dystrophin.13PubMed Central. Pharmacological Profile of Viltolarsen for the Treatment of Duchenne Muscular Dystrophy: A Japanese Experience Additional drugs targeting exon 45 (casimersen) and exon 44 (not yet approved at the time of the most recent sources) have followed the same blueprint, each addressing a different subset of patients based on their specific deletions.
Multi-Exon Skipping
Many patients need more than one exon removed to restore the reading frame, depending on the size and location of their deletion and the base-pair arithmetic of adjacent exons. Multi-exon skipping uses cocktails of AONs to remove two or more exons simultaneously. In theory, this approach could treat roughly 90% of patients with deletion mutations, about 80% with duplications, and approximately 98% with nonsense mutations.14PubMed. Skipping multiple exons of dystrophin transcripts using cocktail antisense oligonucleotides Multi-exon skipping could also benefit patients whose in-frame deletions produce a dystrophin that works poorly, allowing production of a more optimally shortened protein.15PubMed Central. Multiple Exon Skipping in the Duchenne Muscular Dystrophy Hot Spots: Prospects and Challenges
The challenge is delivery. Getting one AON into muscle tissue at therapeutic concentrations is already difficult; getting two or three in the right ratio and into the same cells adds layers of complexity. Multi-exon skipping remains largely preclinical or in early-stage trials for most combinations.
Gene Editing at the Exon Level
CRISPR-Cas9 gene editing offers a fundamentally different way to manipulate exons. Rather than temporarily masking exons from the splicing machinery (as AONs do), gene editing permanently removes a targeted stretch of genomic DNA. Researchers have used pairs of guide RNAs to direct Cas9 to intronic regions flanking a problematic exon, cutting out the exon entirely. In patient-derived muscle cells, CRISPR deletion of exon 51 produced the expected genomic rearrangement and was confirmed by sequencing.16nature communications. Multiplex CRISPR/Cas9-based genome editing for correction of dystrophin mutations that cause Duchenne muscular dystrophy
An ambitious variation targets the entire exon 45–55 region. Guide RNAs aimed at introns 44 and 55 can direct Cas9 to excise tens of thousands of base pairs, fusing the 3′ end of intron 44 with the 5′ end of intron 55 to create a chimeric intron. The resulting transcript joins exon 44 directly to exon 56, producing an in-frame mRNA that encodes a heavily truncated but potentially functional dystrophin.17Cell Stem Cell. A Single CRISPR-Cas9 Deletion Strategy that Targets the Majority of DMD Patients Restores Dystrophin Function in hiPSC-Derived Muscle Cells The appeal of this strategy is breadth: a single guide RNA pair could address the majority of deletions in the central hotspot.
In mouse models, a muscle-specific CRISPR approach targeting the introns flanking exons 52–53 removed a genomic region containing a nonsense mutation while preserving the reading frame. The resulting dystrophin lacked 110 amino acids in a non-essential portion of the protein. A separate strategy in the same study aimed to directly correct small mutations in essential domains, such as the dystroglycan-binding domain, using homology-directed repair to restore the full-length protein.18Nature Communications. Muscle-specific CRISPR/Cas9 dystrophin gene editing ameliorates pathophysiology in a mouse model for Duchenne muscular dystrophy These approaches are still preclinical, but they illustrate how gene editing could tackle mutations that exon skipping cannot easily address.
Micro-Dystrophin and the Limits of Exon-Based Approaches
Exon skipping and exon deletion both produce a protein whose final shape depends on the patient’s underlying mutation. Two patients on the same exon-skipping drug may end up with slightly different truncated dystrophins if their background deletions differ. Gene therapy using micro-dystrophin takes a different approach: it delivers a standardized, rationally designed mini-gene encoding a heavily shortened dystrophin that retains the most important functional domains. Every patient receives the same optimized protein, bypassing the variability inherent in exon-skipping strategies.19PubMed. Clinical potential of microdystrophin as a surrogate endpoint
Micro-dystrophins typically include the N-terminal actin-binding domain, a subset of spectrin-like repeats selected for their binding activities, and the cysteine-rich domain that anchors the protein to the membrane complex. The trade-off is that the protein is much smaller than even a mildly truncated Becker-type dystrophin, and researchers are still working out how well it replaces the full-length version over a patient’s lifetime.
Shorter Isoforms and Effects Beyond Muscle
The full-length dystrophin (called Dp427 for its approximate molecular weight in kilodaltons) is not the only protein produced from the DMD gene. Shorter isoforms are transcribed from internal promoters scattered along the gene’s length, each producing a dystrophin variant that includes only the exons downstream of that promoter. The most studied shorter isoforms include Dp260 (expressed in the retina), Dp140 (brain, kidney), Dp116 (peripheral nerves), and Dp71 (widely expressed, including in the brain).
This has a direct clinical consequence: mutations that fall further along the gene toward its 3′ end tend to knock out more of these isoforms. A patient whose deletion wipes out only the exons encoding Dp427 loses muscle dystrophin but keeps the brain isoforms. A patient whose deletion extends into the region encoding Dp140 or Dp71 may experience cognitive difficulties in addition to muscle disease. Research has found a statistically significant relationship between loss of the Dp140 transcription unit and intellectual disability in both Duchenne and Becker patients.20Neuromuscular Disorders. Loss of Dp140 regulatory sequences is associated with cognitive impairment in dystrophinopathies Patients with normal intelligence generally did not show deletions in the Dp140 regulatory regions, regardless of whether they had Duchenne or Becker.
More granular analysis has shown that the effect on cognition increases as more isoforms are disrupted. Patients with cumulative loss of Dp427, Dp260, and Dp140 had significantly lower IQ scores than those who lost only Dp427.21PubMed Central. Mutation Location and Cognitive Impairment in Duchenne Muscular Dystrophy A larger study grouped patients by the number and type of isoforms affected and found that mutations in the promoter and coding regions of Dp140 had a more profound effect on IQ than mutations hitting only the 5′ untranslated region of that isoform.22PLoS ONE. Dystrophin Gene Mutation Location and the Risk of Cognitive Impairment in Duchenne Muscular Dystrophy For families navigating a new diagnosis, the location of the mutation within the gene’s 79 exons shapes not just the muscle prognosis but expectations for cognitive development as well.
Hidden Mutations and Pseudoexons
Not every disease-causing change in the DMD gene is a classic exon deletion or duplication. Some patients have dystrophin deficiency confirmed by muscle biopsy, yet standard genetic testing for deletions, duplications, and small variants comes back negative. In a subset of these cases, the culprit is a deep intronic variant, a change buried within one of those enormous introns that creates a new splice site. The cell’s splicing machinery mistakenly recognizes a segment of intron as an exon, inserting it into the mRNA. These “pseudoexons” can introduce premature stop codons that destroy the reading frame, effectively causing Duchenne from a mutation invisible to conventional exon-focused testing.23PubMed. A novel deep intronic variant in the DMD gene causes Duchenne muscular dystrophy by pseudoexon activation encoding a nonsense codon
In one family, a deep intronic variant activated a pseudoexon that dominated the mRNA landscape, accounting for roughly 89% of DMD transcripts in cultured muscle cells and about 97% in muscle tissue itself, correlating with near-complete loss of dystrophin.24PubMed. Pseudoexon activating by a deep intronic variant and phenotype variation in a Chinese family with dystrophinopathy These cases highlight why RNA-level analysis from muscle biopsy tissue can uncover diagnoses that DNA-only testing misses. They also represent a potential therapeutic opportunity: an AON designed to block recognition of the pseudoexon could, in theory, silence it and restore normal splicing.
Pinpointing the Mutation and Mapping Exons
Accurate identification of exactly which exons are affected is essential, both for prognosis and for determining eligibility for exon-skipping therapies. Multiplex ligation-dependent probe amplification (MLPA) has become a standard diagnostic tool, capable of detecting deletions and duplications across all 79 exons in a single assay. Compared to older multiplex PCR methods, MLPA picks up additional mutations that would otherwise be missed and defines the boundaries of deletions and duplications more precisely, which directly improves the ability to predict clinical severity.25PubMed. Detecting exon deletions and duplications of the DMD gene using Multiplex Ligation-dependent Probe Amplification (MLPA) More recently, next-generation sequencing has been combined with MLPA to capture not only large structural rearrangements but also small insertions, deletions, and point mutations, including some deep intronic variants.26PubMed Central. Spectrum and carrier frequency of DMD in Yueyang, China: a population-based analysis using NGS and MLPA
Getting the exon map right matters enormously for therapy. A patient told they have a deletion of exons 49–50 is eligible for different skipping strategies than a patient missing exons 49–52. If the deletion boundaries are drawn one exon too narrow or too wide, the predicted reading frame restoration may not work. This is one reason clinical trials for exon-skipping drugs require confirmation of the specific mutation through molecular testing before enrollment.27PubMed. Eteplirsen treatment for Duchenne muscular dystrophy: Exon skipping and dystrophin production
Measuring How Much Dystrophin Is Actually Produced
Whether a therapy is working comes down to one deceptively simple question: how much dystrophin is the patient’s muscle making? Answering it reliably is harder than it sounds. Dystrophin is a low-abundance protein even in healthy muscle, and detecting small increases in patients who started at essentially zero requires sensitive and reproducible methods.
Mass spectrometry has emerged as a leading quantitative approach. One method uses a stable-isotope-labeled dystrophin protein spiked into the sample as an internal standard, combined with gel electrophoresis and high-precision mass spectrometry to measure multiple dystrophin peptides within a complex tissue mixture.28PubMed Central. Accurate Quantitation of Dystrophin Protein in Human Skeletal Muscle Using Mass Spectrometry A complementary immunoaffinity-based mass spectrometry assay has been validated for both full-length dystrophin and mini-dystrophin in human and preclinical samples, achieving sufficient sensitivity for use in clinical trials.29Gene Therapy. Dystrophin and mini-dystrophin quantification by mass spectrometry in skeletal muscle for gene therapy development in Duchenne muscular dystrophy
On the imaging side, automated immunofluorescence methods can scan entire muscle cross-sections and quantify dystrophin intensity without relying on a human operator’s subjective judgment, removing a significant source of variability in traditional pathology scoring.30PLoS ONE. A novel high-throughput immunofluorescence analysis method for quantifying dystrophin intensity in entire transverse sections of Duchenne muscular dystrophy muscle biopsy samples The development of these quantification tools matters for regulatory approval, since demonstrating meaningful increases in dystrophin is a key endpoint for exon-skipping and gene therapy trials alike.
Immune Responses to Newly Made Dystrophin
A concern that sometimes gets overlooked in discussions of restored dystrophin: if a patient’s immune system has never encountered the protein, will it treat newly produced dystrophin as foreign? In dystrophic mice treated with morpholino-induced exon skipping, researchers observed both antibody-mediated and T-cell-mediated immune responses directed against the de novo dystrophin. Some mice developed circulating antibodies against the restored protein, and those antibody-positive animals also showed stronger antigen-specific T-cell responses. Immune cell infiltration and markers of cell-mediated damage were found near dystrophin-expressing muscle fibers.31PubMed Central. Morpholino-induced exon skipping stimulates cell-mediated and humoral responses to dystrophin in mdx mice
The finding raised questions about the long-term durability of any dystrophin-restoration strategy. If the immune system mounts an ongoing attack against treated muscle fibers, the therapeutic benefit could erode over time. The study underscored the need to investigate autoimmune responses against truncated dystrophin and their long-term consequences in clinical settings.32PubMed. Autoimmune response and its long-term consequences after exon-skipping therapy in a Duchenne muscular dystrophy mouse model Whether this plays out the same way in humans, whose immune tolerance to dystrophin fragments may differ from that of laboratory mice, remains an open question. Some patients do produce small amounts of dystrophin from naturally occurring “revertant” fibers, which could prime a degree of immune tolerance. But for patients with no detectable dystrophin at all, the immune landscape after treatment is something clinicians are watching closely.
From Gene Discovery to Exon-Level Medicine
The first fragments of DMD gene cDNA were identified in 1986, and the full 3,685 amino acid sequence of dystrophin was predicted the following year, along with production of the first antibodies against the protein.33PubMed Central. The discovery of dystrophin, the protein product of the Duchenne muscular dystrophy gene In the roughly four decades since, the field has moved from simply knowing the gene existed to designing therapies that manipulate individual exons. That progression reflects an increasingly fine-grained understanding of which parts of the protein matter, which can be safely removed, and how the cell’s own splicing machinery can be co-opted or permanently rewritten to produce a functional product. The 79 exons of the DMD gene are no longer just a structural curiosity of the longest human gene. Each one is a potential therapeutic lever, and matching the right intervention to the right exon is where much of the field’s energy is now focused.