Moira MacTaggert met Charles Xavier on a post-graduate genetics course in Glasgow, and,after a three-week whirlwind romance,they married. After four years they were blessed with a son, David. The couple pioneered mutant research, and Moira designed the mutant detection device Cerebro, which they used to track down young mutants who needed medical care. Eventually, their work attracted the attention of adult mutant Erik Lensherr; seduced by their common dream of a new mutant society, Charles abandoned his family to help Erik build that society. The next day David's own mutant powers emerged, and he had to be sedated to prevent those powers from killing him.
A couple of years ago, Charles got back in touch with Moira. He had broken ties with Erik, and was setting up a mutant school in the U.S.A.; he also intended to establish a secret mutant hospital, run by Moira, in part as a way of ensuring David got the best medical attention. Moira accepted, setting up base on Muir Island off the North coast of Scotland.
When David awoke and went on a killing spree, Moira called in Charles and his students, the X-Men. They pursued David to Aberdeen and then Berlin, where he was apparently slain. When the X-Men became fugitives for hiding Magneto's survival from the authorities, Moira went into hiding with her charges. After the X-Men captured Magneto, Moira returned to Muir Island.
MacTaggert recently returned as a pivotal player in the Banshee issues. She had been manipulating Wolverine's DNA to make the power-giving drug known as Banshee, possibly named due to the fact that the drug gave her the ability to project super-sonic screams. She had been distributing Banshee so as to fund her Muir Isle. Research Center. After a confrontation with Wolverine, resulting in the latter of which blowing up the entire Banshee store, it was revealed that MacTaggert had been working with Magneto.