Dystrophin

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.

As of 2007 dystrophin has the longest gene known, at locus Xp21. The primary transcript measures 2.4 megabases (thus the gene comprises 0.008% of the human genome), and takes 16 hours to transcribe. The 79 exons code for a protein of over 3500 amino acid residues.

Pathology
Its deficiency is one of the root causes of muscular dystrophy. It was first identified in 1987 by Louis M. Kunkel , after the 1986 discovery of the mutated gene that causes Duchenne muscular dystrophy (DMD) .

Normal tissue contains small amounts of dystrophin (about 0.002% of total muscle protein), but its absence leads to both DMD and fibrosis, a condition of muscle hardening. A different mutation of the same gene causes defective dystrophin, leading to Becker's muscular dystrophy (BMD).