The US companies being hit hardest by Brexit vote

Thursday's stunning Brexit vote threw global markets into a tailspin, as investors reacted to the unprecedented uncertainty of a country pulling out of the European Union. The FTSE 100 fell as much as 8 percent. The S&P 500 fell 2.4 percent in the first hour of trading.

The British referendum could seem to Americans like a provincial problem. But from the perspective of many American companies, the British are employees and customers. The vote to leave the EU could have a significant effect on corporate America's bottom line.

A number of companies in the S&P 500 have significant revenue exposure in the U.K., and investors are noticeably nervous.

Among the most exposed companies are global brands like Molson Coors, BlackRock and eBay. Each of them get more than 15 percent of their revenue from the United Kingdom.

As a broad sector, energy names in particular are heavily exposed to the U.K. Newmont Mining generated 64 percent of its revenue in the U.K., the highest figure among the S&P 500. Bucking the trend of exposed companies' poor performance, the company's stock is leading the index, up 5.3 percent today. Newmont is likely benefiting from its gold mining operations as investors seek safe havens from the otherwise volatile markets.

Apache Corp., Transocean, ConocoPhillips and Marathon Oil Corp. are not as lucky. The oil and gas companies are exposed to U.K. markets and have no security like gold. The energy sector has already been hit hard in the past year by falling oil prices and shifts in fossil fuel demands.

All energy stocks in the S&P 500 were down as of 10:30 a.m. ET.

Financials and energy companies led the index lower, averaging declines of 3.6 percent and 3.5 percent, respectively. Invesco, which generates 23 percent of its revenue in the U.K., fared the worst, down 10 percent after an hour of trading. E*Trade, Morgan Stanley and Citigroup were also down around 8 percent.

Utilities were the only sector in the black an hour after the markets opened. On average, utility stocks were up just 0.4 percent, led by CMS Energy Corp. with +2.1 percent.

On the flip side, there are companies with very little "Brexposure." Think of the very traditional brands: AT&T, J.M. Smucker and Best Buy earn effectively nothing from the U.K.: less than 0.2 percent of their revenue.

More broadly, domestic U.S. insurance companies are also immune to U.K. exposure. Those include UnitedHealth Group, Cardinal Health and Aetna.

But even some of the very least U.K.-exposed companies in the S&P 500 took a tumble on Friday morning. Among the 20 with the least revenue coming from the U.K., prices fell a median of 2 percent, compared with a 4.3 percent decline for the most exposed.

It's unclear how the Brexit vote could affect companies that derive significant revenue from the country, but experts predict a coming period of volatility at the very least.

JPMorgan Chase's Jamie Dimon has said that a vote to leave the EU could threaten up to 4,000 jobs in the country that support the bank's European business. JPMorgan employs some 16,000 people in Britain, according to Reuters. Toyota has said that a Brexit could lead to 10 percent duties on its cars built in the U.K.

Overall, around 2.9 percent of the S&P 500's total revenue comes from the U.K. So if you just owned the S&P 500 overall, there is very little exposure to Brexit. For the most part, the drama has been about what this means in a macro sense, even though the largest American companies have twice as much Chinese exposure as they do British.