Not much is known about Cain Marko. All that is known is that he was incarcerated by Weapon X and forced to act as a living weapon under the direction of Col. John Wraith. Juggernaut was part of the strike force that took out the X-Men, forcing them into Weapon X as well. When The Brotherhood of Mutants removed the security implants that were prohibiting the mutants from leaving their cells, Juggernaut fought for his freedom. After the entire ordeal, Cain was offered a place with both Professor X's X-men and Magneto's Brotherhood. Cain chose The Brotherhood, but has apparently left the team for parts unknown. Recently, Juggernaut has bonded with a gem stolen by Rogue and Gambit called the Cyttorak Gem. Though its full repercussions are unknown, Juggernaut did describe himself as feeling "better than ever."
When fully powered with the gem, Cain confessed his feelings for Rogue, telling her that he fell in love the first time they met. Rogue did not want this and tried to drain his powers but was easily taken down. Gambit fought Cain as much as he could, ending the battle by charging a card and throwing it at a building, making it collapse on them both. While Gambit is known to have died, Cain may have survived.