U.S. stocks were lower a day ahead of a Federal Reserve policy meeting as Microsoft shares dragged all three major indexes down after the company said it would buy LinkedIn for $26.2 billion.
Microsoft fell as much as 4.7 percent to a four-month low of $49.05. LinkedIn jumped 46.8 percent to $192.43.
The Federal Reserve Open Market Committee (FOMC) will meet on Tuesday and Wednesday to decide when to raise interest rates for the second time in nearly a decade.
Fed Chair Janet Yellen, who had been dropping hints of a rate hike most of last month, was more vague last week on its timing after dismal May hiring data and a possible Brexit raised concerns about the strength of the economy.
"Today seems to be a very volatile day.... The market is always looking for any message the Fed may have in terms of what they do in July or what the statement may be for the meeting," said Omar Aguilar, chief investment officer at Charles Schwab Investment Management.
At 12:24 a.m. ET (1624 GMT), the Dow Jones Industrial Average was down 52.87 points, or 0.3 percent, at 17,812.47.
The S&P 500 was down 7.57 points, or 0.36 percent, at 2,088.5.
The tech-heavy Nasdaq Composite was down 29.79 points, or 0.61 percent, at 4,864.76.
Nine of the 10 major S&P indexes were lower, led by the 0.92 percent fall in the information technology index.
The Brexit vote provides a lot of volatility, especially in the currency markets, and will continue to do so for the next two weeks, Aguilar said.
Britain will vote on June 23 to determine if it would remain in the European Union.
Safe-haven assets soared on the uncertainty.
Gold prices hit their highest since mid-May and the yen surged to a three-year high.
Oil prices, which showed a momentary rise earlier in the day, were down about 0.5 percent.
An attack in Orlando, Florida, where a gunman killed over 50 people at a nightclub added to the dour mood. Shares of gun makers Smith & Wesson rose 7 percent and Sturm Ruger rose 9 percent.
Apple, which kicked off its annual developers' conference, was down 1.4 percent at $97.42.
Facebook was down 2.7 percent at $113.37 after CNBC said Citron Research was short on the stock.
Declining issues outnumbered advancing ones on the NYSE by 1,831 to 1,102. On the Nasdaq, 1,805 issues fell and 924 advanced.
The S&P 500 index showed 24 new 52-week highs and 2 new lows, while the Nasdaq recorded 34 new highs and 44 new lows. (Reporting by Yashaswini Swamynathan in Bengaluru; Editing by Don Sebastian)