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I don't know that Poloniex has any plans to add USD, but you should absolutely keep an eye on @bittrex. They have made it very clear that they are in the works of adding USD along with a new and very much improved UI design to the site.

@bittrex-bill and @bittrex-richie have both made statements about it and they have been in the process of taking care of all legalities to get USD for months now.

Aren't these just pass-thru transfers to forex exchanges?

Not likely. I think they'll actually be a USD/BTC exchange.

Don't expect an announcement. it'll be a surprise you'll wake up one morning and boom there it is!!!

Just a thought but Poloniex has tether (usdt) which is a form of usd if I'm correct

Polo does only do crypto and I hope they keep it that way.