The project is working under the codename "143."
Deadpool director Tim Miller and Brian Michael Bendis, one of the biggest authors in the comic book world, are teaming for an X-Men spinoff.
The duo are developing a project called 143, which will be set in the X-Men universe, The Hollywood Reporter has confirmed. Bendis, who wrote various X-Men comics during his almost 20 years at Marvel, will write the script, while Miller will direct.
143 is a code name for the project which sources say will focus on the character Kitty Pryde, a mutant who can phase through objects. The character was played by Ellen Page in 2006's X-Men: The Last Stand and 2014's X-Men: Days of Future Past.
The project's code name is a reference to Uncanny X-Men No. 143, one of the earliest Pryde stories and her first solo adventure in which she faces a demon alone in the X-Mansion. The issue, by her creators Chris Claremont and John Byrne, was published in 1981.
Pryde has long been an X-Men fan favorite and is influential, serving as an inspiration for Joss Whedon's Buffy the Vampire Slayer.
Collider.com initially wrote about Miller developing the project, for which Bendis was just hired to work on.
Miller, repped by WME, broke out as a director with 2016's Deadpool, which went on to become the highest-grossing X-Men film ever with $783 million worldwide.
Bendis, repped by Circle of Confusion, surprised the comics industry last year when he joined DC after spending 17 years as a top-billed writer for Marvel, where he created characters such as Miles Morales and Jessica Jones. He also was part of the Marvel Creative Committee that advised Marvel Studios' output through 2015.