Twitter co-founder Evan Williams, who’s currently CEO of publishing platform Medium, is stepping down from Twitter’s board, according to a new filing with the Securities and Exchange Commission. Williams will officially depart at the end of the month.

“It’s been an incredible 13 years, and I’m proud of what Twitter has accomplished during my time with the company,” Williams said in a statement from the filing. “I will continue rooting for the team as I focus my time on other projects.”

I'm very lucky to have served on the @Twitter board for 12 years (ever since there was a board). It's been overwhelmingly interesting, educational—and, at times, challenging. — Ev Williams (@ev) February 22, 2019

Williams co-founded Twitter in 2007 with Jack Dorsey and Biz Stone after the social network spun out from other projects that the group worked on together at podcasting company Odeo. In 2008, Williams became the CEO of Twitter, replacing Dorsey, and was later replaced in 2010 by Dick Costolo (who would later be pushed out by none other than Dorsey), as documented by Nick Bilton’s betrayal-filled tell-all Hatching Twitter. Despite all the backstabbing, Williams has served on Twitter’s board from the beginning, totaling 12 years.

Soon after news broke, Williams sent out a tweet thanking his colleagues. “Thank you, @jack and @biz for starting this crazy company with me—and continuing to make it better and better. And to my fellow board members, new and old—some of the most thoughtful people I’ve ever known,” he said. “I’m very lucky to have served on the @Twitter board for 12 years (ever since there was a board). It’s been overwhelmingly interesting, educational—and, at times, challenging.”

Twitter did not respond to a request for comment.