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Revlimid is Celgene's flagship blood cancer drug and is already approved for use in various types of blood cancer, including multiple myeloma and mantle cell lymphoma.

Celgene shares were down 2 percent to $134 in premarket trading.

The trial was placed on a clinical hold by the U.S. Food and Drug Administration on July 12, Celgene said. The study was testing the drug in previously untreated, elderly patients with B-cell chronic lymphocytic leukemia.

There were 34 deaths in the 210 patients who were treated by Revlimid, compared to 18 deaths in the 211 patients receiving a chemotherapy drug. The specific reason for the imbalance has not yet been identified, Celgene said.

Other trials testing Revlimid in chronic lymphocytic leukemia are continuing. However, all clinical investigators in those studies will be officially advised of the trial halt and instructed to inform their patients.

Revlimid, Celgene's most important growth driver, brought in sales of about $1 billion during the first quarter.

Last week, the company reported positive data from another late-stage Revlimid study that can potentially open up a wider use of the drug.

(Reporting by Esha Dey in Bangalore; Editing by Sreejiraj Eluvangal)