SAN FRANCISCO (AP) — Twitter co-founder Evan Williams will leave the social media company’s board of directors at the end of the month.

Twitter announced his departure in a securities filings Friday and Williams tweeted confirmation of the news.

Williams served as Twitter’s CEO from 2008 to 2010 and now is chief executive of publishing site Medium.com. He tweeted that stepping down will let him focus on other projects and that he will still be rooting for the Twitter team.

He was the second-largest Twitter shareholder on the company’s board behind CEO and co-founder Jack Dorsey.

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Dorsey tweeted his appreciation for Williams, who served on the board for 12 years.