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R&B singer D'Angelo is calling off a string of concerts due to a "medical emergency." 

According to the "Chicago Tribune," D'Angelo has canceled his upcoming concerts in Chicago and San Diego. 

Concert promoter Live Nation says D'Angelo "expresses his sincere regret due to this unfortunate circumstance." 

While the nature of D'Angelo's "medical emergency" has not been made public, an executive from Chicago's House of Blues - where D'Angelo was set to play this week -calls the emergency "serious," but says he "should be OK."  

D'Angelo is scheduled to take the stage at San Francisco's Outside Lands Music Festival this weekend.  

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