Stocks: 5 things to know before the open

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Recent market swings are enough to give anyone indigestion.

U.S. stock futures are down by about 1% ahead of the open, Asian markets ended Wednesday with deep losses, but in Europe most of the major markets are edging up in early trading.

Here are the key things you need to know before the opening bell rings in New York:

1. Keep an eye on China: It's impossible to ignore what's happening in the world's second-largest economy right now.

A crisis of confidence has wiped trillions of dollars off the country's stock markets.

The Shanghai Composite lost another 5.9% Wednesday. The smaller Shenzhen Composite lost 2.5%, while Hong Kong's Hang Seng dropped 5.8%.

To put this in perspective, the Shanghai market has dropped by over 30% since hitting a recent peak and the Shenzhen is off by 40%.

"At the moment there is a mood of panic in the market and a large increase in irrational dumping of shares, causing a strain of liquidity in the stock market," China Securities Regulatory Commission said in statement.

More than half of all listed Chinese companies have opted to suspend trading in their shares in what is likely a defense against volatility. But it's possible that they are only delaying the inevitable.

Related: Fear & Greed Index

2. What about Greece?: Ah yes. Greece. The crisis continues as banks are still shut, the flow of money has nearly run dry and the government is butting heads with other European leaders as it tries to arrange a third bailout package.

European markets have taken a collective tumble over the past couple weeks as traders worry about the potential fallout if Greece has to drop out of the euro and begin printing its own currency.

But right now, markets are relatively stable as they await further bailout negotiations in the coming few days.

Related: The Greek crisis in 2 minutes

3. U.S. market overview: U.S. stock futures are indicating there will be a significant drop at the open.

Premarket data shows the following companies could see their shares tumble when trading begins: Ross Stores (ROST), Freeport-McMoRan (FCX), Alcoa (AA) and Yahoo (YHOO). The shares in these firms are all off by over 2% in extended trading.

Looking back at Tuesday, the Dow Jones industrial average gained 93 points, up by 0.5%. The S&P 500 rose 0.6% and the Nasdaq inched up by 0.1%.

4. Barclays ditches CEO: Shares in the British bank Barclays (BCS) are rising by about 3% in London after the company announced it had ousted chief executive Antony Jenkins.

Chairman John McFarlane will act as executive chairman until a new CEO appointment is made.

The bank said the move doesn't signal a major change in its strategy, but that fresh leadership was needed.

5. Earnings and economics: The Federal Reserve committee that helps set monetary policy will release the talking points from its latest meeting at 2 p.m. ET. The meeting minutes are closely monitored so traders can get an indication of when the Fed will decide to raise interest rates.

On the earnings side, Alcoa (AA) is reporting earnings after the markets close.

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