A pile of Bitcoins are shown here after Software engineer Mike Caldwell minted them in his shop on April 26, 2013 in Sandy, Utah. - 

U.S. officials have been warning for months that the unregulated digital currency Bitcoin wasn't fully legitimate in the eyes of the law. Now, the Department of Homeland Security has frozen an account for one of the largest Bitcoin exchanges, a firm called Mt. Gox. The online firm, based in Tokyo, says it handles nearly 80 percent of global trading for this form of virtual cash.

Queena Kim, Marketplace Tech reporter, explains the latest on the story.