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The Joker's on the loose.
Nobody's quite sure what his real identity is, or where he lives. No one's ever seen him, heard him, or managed to track him down.
But one thing's clear: He's pissed a lot of people off.
"He's a f-----" one Tehran-based street artist wrote to me. "He is doing something that nobody should do... he is not an important one to talk about."
The Joker has been running around Tehran spraying on other people's graffiti and stencils, violating the biggest unwritten rule of street art: Don't mess with another artist's work.
Several works around the city have been defaced with a his trademark red smile painted on top of existing faces, with the word's "Joker" scribbled next to it.
The effect looks like lipstick, similar to the way Joker from the comic books would have written his name.
What makes the Joker's work more mysterious isn't just what he does, but how quickly he does it. In a city where street art can last for just a few days before city workers paint over it, the Joker has been able to get to and deface some pieces before city officials find out where the pieces are.
It has some in the fledgeling street art community worried that the Joker's real identity might be an even bigger threat: He could be one of their own.