The X-Men's first major mission was to retrieve the stolen Cerebro machine from the Leader. On this mission Xavier recruited two new mutants to be X-Men, Sunfire and Boom-Boom (although she ran away shortly after this mission). The group succeeded in defeating the Leader, but had to travel to Europe to obtain Cerebro. They were ultimately able to get Cerebro following an altercation between a group of terrorist mutants led by the mysterious Fantomex. The Russian mutant Colossus was a member of the terrorist group, but he decided to help the X-Men on their mission because he disliked the violent nature of the terrorist group. He then became an official X-Men member.
When the X-Men Mansion was attacked by the powerful alien named Tyros, they were able to defeat him; however, when he returned as Terrax, it was only thanks to the Silver Surfer that Terrax left without killing the X-Men.