Another key product exec is leaving Twitter.

Trevor O’Brien, who joined Twitter a year and a half ago from YouTube and oversees product for Twitter’s iOS and Android apps, is departing the company, according to multiple sources. O’Brien also oversaw Twitter’s timeline product at one point, and worked on the efforts to put tweet and follower recommendations into users’ timelines.

A company spokesperson confirmed O’Brien’s departure.

It’s unclear where he’s headed. O’Brien joined Twitter in late 2013 after almost four years at Google, including a nearly three-year-long stint running search and discovery for YouTube, according to his LinkedIn profile.

O’Brien’s departure was announced internally earlier this week alongside those of fellow product execs Christian Oestlien and Todd Jackson, according to sources. Oestlien and Jackson announced on Twitter that they were leaving at the same time Twitter reported earnings on Tuesday. The trio of departures is a major blow to the company and specifically the product team, which was essentially called out by Interim CEO Jack Dorsey earlier this week on the company earnings call.

Twitter’s stock is down more than 15 percent since the close of market Tuesday.