Michelle Obama celebrates dog Bo's birthday

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Bo Obama, who moved into the White House in April 2009, turned 4 on Tuesday.

The dog, who the first lady has called "her son," stars in his own campaign ads and official, pro-Obama merchandise. The family dog may not be on Twitter, but his fans are; photos of the photogenic Bo with the Obama family are typically well-received online.

On Monday, Michelle Obama tweeted an old photo of Bo's brother Cappie, who belongs to the couple that gave Bo to the Obamas: the late Sen. Ted Kennedy (D-Mass.) and wife Vicki.