The Chicago Mercantile Exchange, following its threats to leave the city over a tax increase, has put its building up for sale, according to the Chicago Sun-Times. The sale does not necessarily signal the CME is ready to leave — it will essentially lease back a large portion of the building from the future buyer. The building is the national landmark Chicago Board of Trade Building. The CME will lease back space for at least 15 years, including the trading floor where agricultural commodities are traded. No word on an asking price yet.