This is the core team that Cyclops assembled right after the split of the X-men during the Schism, with Wolverine leading the other half. The Extinction team contained the most powerful mutants Cyclops had in his group, which he planned to use as a reminder to the mutant haters that the X-men had weapons of mass destruction.
Also, Mister Sinister is not part of the group. He was simply the team’s nemesis in their first field operation.
From – Uncanny X-Men Vol. 2 #1