Those who’ve read the comics know that the two issue story, taking place in Uncanny X-Men #141 and 142 way back in 1981 during Chris Claremont and John Byrne’s run, introduced the idea of an alternate future for Marvel’s mutants that comes out of the Brotherhood of Evil Mutants killing an important senator, leading to a future where all mutants are hunted by Sentinels.
If this is in fact the plan, this opens up a lot of intriguing ideas since the original two-part story heavily featured Wolverine (who only cameod in X-Men: First Class) and an older version of Kitty Pryde, who appeared in X-Men: The Last Stand, as played by Ellen Page, but not in First Class either. The storyline also introduced Rachel Summers, an alternate future spawn of Scott Summers and Jean Grey, and the ultimate Sentinel called Nimrod.
The choice to explore this storyline is also interesting since writer Simon Kinberg recently hinted to SuperHeroHype that the sequel would focus on the relationships between Professor X and Magneto, as played by James McAvoy and Mike Fassbender, and Mystique and Hank McCoy, as played by Jennifer Lawrence and Nicholas Hoult, and that they’d retain the period aspect of First Class. With that in mind, we can probably assume that the sequel won’t be a direct adaptation of the Claremont/Byrne story but will use elements from that storyline.
