Where does <div id=”_mcePaste”> come from?

I’ve purchased a WP theme
When I paste text and edit, it all seems to come out funny

When I look at the HTML of the text, it’s got <div id=”_mcePaste”> all over the place

I’ve done a grep of all PHP and JS files for ‘_mcePaste’ – it’s not in any of the files

So: it can’t be a problem with the theme or plugin code?

But: I’ve never had the same problem with other themes?

Does anyone know about this problem?
Or is it just me!?

Thanks

Omar