Help with if then statement

Okay, so I'm putting this into the frames action, and I have a movie clip named "mc" on the stage"

code:

asdf = setInterval(chek, 10);

function chek(){
if (_root.mc._x<_root._xmouse) then (_root.mc._x+20)
}


I say check for errors and none come up, but when I test movie, I move my mouse further than "mc" and the movie clip doesn't move, why?
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