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(Reuters) - HBO Chief Executive Richard Plepler will leave the premium television network owned by U.S. wireless carrier AT&T Inc after nearly 28 years at the company, Plepler said in a memo to HBO employees on Thursday.

Turner President David Levy is also expected to resign from the broadcasting company, which is also owned by AT&T, according to a source familiar with the plan.

AT&T unit WarnerMedia, which oversees HBO and Turner, declined to comment.

“Hard as it is to think about leaving the company I love, and the people I love in it, it is the right time for me to do so,” Plepler wrote in the memo seen by Reuters.

Plepler oversaw many successes at HBO such as hit shows like “Game of Thrones” and “Girls.”

The news comes after AT&T won an appeals court decision on Tuesday for its purchase of media company Time Warner, which included HBO and the Turner TV networks.

Many in the industry had expected AT&T would begin to restructure the HBO and Turner properties after its second victory over the U.S. Department of Justice, which had sued to stop the merger.

Former NBC Chairman Robert Greenblatt is in talks to take a leadership role at WarnerMedia, the source added.