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DMD is the biggest gene humans have, spanning over two million letters of DNA. It makes dystrophin, a long springy protein that anchors the inside of a muscle cell to its outer membrane. Without it, every contraction tears the cell a little, and muscle is gradually lost. Its enormous size is a large part of why treating it is so hard: it will not fit into standard delivery vehicles.
DMD, at Xp21.2-p21.1, spans approximately 2.2 million base pairs with 79 exons and encodes dystrophin, which links the actin cytoskeleton to the extracellular matrix through the dystrophin-glycoprotein complex. Out-of-frame mutations abolish the protein and cause Duchenne muscular dystrophy; in-frame deletions produce a truncated but partly functional protein and the milder Becker phenotype — the observation that underlies exon-skipping strategies.
Sources
- MedlinePlus Genetics, U.S. National Library of Medicine
DMD gene ↗