Muir Island's significance stems from the fact that it is the home of Earth's largest and most comprehensive mutant research complex, located to the north of Scotland, founded by Dr. Moira MacTaggert. Originally, she created the facility to help her son, Kevin, an extremely powerful and destructive mutant. Muir Island's facility holds a wealth of knowledge on various mutants, their abilities, and other mutant-related issues and concerns (such as the Legacy Virus). It was also once the home base of the X-Men's spin-off superhero team Excalibur. It was used by Professor X to store information on how to kill various X-Men if need be. The island has sometimes been used to imprison mutants for various reasons, notably Spoor and Magneto. The staff has included, at times, Rahne Sinclaire, Rory Campbell, Tom Corsi, Sharon Friedlander, Jamie Madrox and, seen in various comics, several un-named scientists. The island and its research center have come under attack by many villains, including Magneto, the Acolytes, the Shadow King, Galactus, Phalanx, Reavers, Black Air, the Mutant Liberation Front, Spiral, Bastion, and Proteus. The island's facilities were destroyed by Legion (who was, at the time, possessed by the Shadow King) during the Muir Island Saga. Although rebuilt, the Muir Island Genetic Research Center was partly destroyed again in an attack by Mystique and Sabretooth. The X-Man Banshee retrieves several needed files from Muir and it is later revealed to be where Professor Xavier had been living after the events of House of M. It was later refurbished for the Marauders to use during X-Men: Messiah Complex. Various X-Men invade and confront the Marauders. When Destiny was resurrected she tried telepathically contacting her foster-daughter Rogue, but instead found Blindfold. There Proteus possessed the young girl and used her to send some of the X-Men to Muir Island where he reveals himself to Rogue.

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