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Boonen stepped off his bike 59 kilometers from the finish of the 15th stage, which covers more than 187 kilometers from Gap to l'Alpe d'Huez.
"Tom does not understand what happened to him," Quick Step team spokesman Alessandro Tegner said by telephone.
"He passed the Col de l'Izoard with the favorites and suddenly, on the descent, he had trouble breathing."
"He could not control it. He lowered his pace and when he was trying to step up a gear, it was worse. He was powerless, he could not go on."
The Belgian Quick Step rider, who was second in the
standings for the best sprinter's green jersey, failed to win a
stage but wore the overall leader's yellow jersey for three