A photo of the defaced painting by Mark Rothko at the Tate Modern
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Mark Rothko painting at Tate Modern defaced

8 October 2012 Last updated at 18:58 BST

A painting by artist Mark Rothko at the Tate Modern has been defaced by a small amount of black paint.

Paintings by the Modernist painter, one of America's most important post-war artists, have sold for tens of millions of pounds.

Will Gompertz reports.