Series began as a vehicle to star the original X-Men team. To that end, Jean Grey had been brought back in a two-part Avengers & Fantastic Four story. For the first 25 issues, the team masqueraded as mutant hunters but revealed themselves to be heroes. Following the Muir Island Saga, the title was retooled with a new cast as a government sponsored team, though they evntually went rogue.
Series ended with #149 to be replaced with Mutant X. X-Factor was originally intended to return following the completion of the Mutant X storyline, but this did not happen. X-Factor (2nd series) would have no connection to this series, but X-Factor (3rd series) was a thematic sequel.
