Meta Platforms‘ CEO Mark Zuckerberg will need to testify at the first trial over the adverse effects of social media on younger users, a Los Angeles Superior Court judge ruled Monday.

Judge Carolyn B. Kuhl also ordered Snap CEO Evan Spiegel and Instagram’s Adam Mosseri to testify at the “first bellweather trial” slated for January. It followed a hearing on Monday in which representatives attempted to prevent the CEOs from testifying.

“The testimony of a CEO is uniquely relevant, as that officer’s knowledge of harms, and failure to take available steps to avoid such harms could establish negligence or ratification of negligent conduct,” she wrote.

Kuhl ruled that their testimonies would be “unique,” given the allegations that social media companies failed to warn users of features created to “be addictive” and “drive compulsive” behaviors in minors.

Law360 was first to report the judge’s order.

Meta declined to comment. CNBC has reached out to Snap for comment.

The pending trial is one of many alleging that social media technology companies have deliberately created platforms and features that cause harm to younger users and damage their mental health.

Earlier this month, New York City filed a lawsuit against several companies, arguing that they created addictive platforms that contribute to mental health issues among children. Last January, CEOs, including Zuckerberg, testified at a Senate hearing on ways to protect children from the harms of social media.

Meta motioned to stop Zuckerberg and Mosseri from testifying at the trial and argued for plaintiffs to use previous trial testimony. The company said making both CEOs attend more in-person trials would interfere with business and “set a precedent” to appear at other related trials.

Snap argued that ordering Spiegel to testify would be an “abuse of discretion.”

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