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A federal judge in San Francisco on Thursday disqualified lawyers at Quinn Emanuel Urquhart & Sullivan from representing defunct ride-hailing service Sidecar in an antitrust lawsuit against the law firm's former client Uber Technologies Inc.

U.S. Chief Magistrate Judge Joseph Spero found that the firm's work defending Uber from predatory pricing claims brought in a 2014 lawsuit from a Maryland taxi company had a “substantial relationship” to the case the firm's lawyers were pursuing on behalf of Sidecar.

Spero wrote that “it is reasonable for Uber to expect that Quinn Emanuel would not now serve as counsel to a plaintiff bringing antitrust claims based on Uber's alleged conduct during the same time that Quinn Emanuel served as Uber's sole outside litigation counsel and defended Uber against unfair competition and antitrust claims—including at least some claims turning on factual questions underlying the case at hand.”

Lawyers at Quinn Emanuel had tried to distinguish the Maryland case from their work for Sidecar by pointing out that the earlier case dealt with Uber's noncompliance with regulated prices for cab fares where the Sidecar case dealt with allegations that Uber was charging below-cost prices aimed at eliminating competition in the market for ride-hailing software. Spero, however, noted in Thursday's opinion that the “the software-only market definition on which Sidecar relies to distinguish [the Maryland case] was a legal strategy that Quinn Emanuel pioneered for Uber in the years in which it served as Uber's litigation counsel.”

Uber's lead outside counsel, Theodore Boutrous Jr. of Gibson, Dunn & Crutcher, referred a request for comment to the company. Uber representatives didn't immediately respond to messages.

Robert Feldman of Quinn Emanuel declined to comment.

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