Lucas Bear Cervone, who battled cancer for most his life, was diagnosed with leukemia at age 2.
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"At 506pm CST, Lucas BEAR took his last breath. He is cancer free and no longer in pain," read the Facebook post.
His family had held a party for him on Saturday because they worried he might not make it to the big day.
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Cervone was flooded with birthday cards in the last week after news spread that cards and letters would help him during a difficult time. The Chicago UPS Store where the cards were delivered was running out of space because so many letters were sent.