<Tr> <Td_colspan="2"> View / Edit Human </Td> <Td_colspan="2"> View / Edit Mouse </Td> </Tr> <P> Dystrophin is a rod - shaped cytoplasmic protein, and a vital part of a protein complex that connects the cytoskeleton of a muscle fiber to the surrounding extracellular matrix through the cell membrane . This complex is variously known as the costamere or the dystrophin - associated protein complex . Many muscle proteins, such as α - dystrobrevin, syncoilin, synemin, sarcoglycan, dystroglycan, and sarcospan, colocalize with dystrophin at the costamere . </P> <P> The DMD gene, encoding the dystrophin protein, is one of the longest human genes known, covering 2.3 megabases (0.08% of the human genome) at locus Xp21 . The primary transcript in muscle measures about 2,100 kilobases and takes 16 hours to transcribe; the mature mRNA measures 14.0 kilobases . The 79 - exon muscle transcript codes for a protein of 3685 amino acid residues . </P> <P> Dystrophin is a protein located between the sarcolemma and the outermost layer of myofilaments in the muscle fiber (myofiber). It is a cohesive protein, linking actin filaments to another support protein that resides on the inside surface of each muscle fiber's plasma membrane (sarcolemma). This support protein on the inside surface of the sarcolemma in turn links to two other consecutive proteins for a total of three linking proteins . The final linking protein is attached to the fibrous endomysium of the entire muscle fiber . Dystrophin supports muscle fiber strength, and the absence of dystrophin reduces muscle stiffness, increases sarcolemmal deformability, and compromises the mechanical stability of costameres and their connections to nearby myofibrils; as shown in recent studies where biomechanical properties of the sarcolemma and its links through costameres to the contractile apparatus were measured, and helps to prevent muscle fiber injury . Movement of thin filaments (actin) creates a pulling force on the extracellular connective tissue that eventually becomes the tendon of the muscle . </P>

The dmd mutation is found where in the human genome