The long-running prosecution of allegedly improper trading at the New York Stock Exchange has seen more guilty pleas vacated, the Wall Street Journal reports.

A federal judge set aside the guilty pleas of two former floor traders at Van der Moolen Specialists USA LLC, and prosecutors dropped the criminal case against a former LaBranche & Co. trader for allegedly making improper trades for their firms' accounts at the NYSE.

The cases centered on the NYSE's "specialist firms," the elite trading operations that are in charge of specific stocks on the floor of the exchange, a unit of NYSE Euronext.

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