S&P 500 futures slump after Britons vote to leave EU

Futures on the S&P 500 and Nasdaq plummeted 5 percent early on Friday and triggered overnight circuit breakers after Britain's vote to quit the European Union catapulted global markets into turmoil. The drastic drops in U.S. stock index futures suggest Wall Street will open with steep losses later in the day, with the historic referendum sparking the biggest global financial shock since the 2008 financial crisis. Britain's FTSE 100 stock index slumped 7 percent in early trade.