From tiny startups to global giants, the companies that sustain the $324 billion U.S. biotech industry are increasingly alarmed as President Donald Trump considers following his controversial travel ban with restrictions on skilled foreign immigrants.

To crank out discoveries, U.S. biotech firms such as Amgen Inc. and Gilead Sciences Inc., as well as overseas companies with stateside operations, rely on the worlds best scientists and lower-level researchers with scarce expertise.

A crackdown on visas for these workers could set back research, including the treatment of cancer, executives said. It also comes as companies, hospitals and universities struggle with the aftermath of Trumps immigration ban from seven Muslim-majority countries, which has for now been blocked in court.

Theres a real crisis of science going on in this country, said Cedric Francois, chief executive officer of Apellis Pharmaceuticals, a startup based in Crestwood, Kentucky, that is working on immune-therapy drugs. About half of Apelliss staff come from abroad, largely brought in on the kind of visas, called H-1B, that allow temporary residence for skilled foreign workers.

Most, if not all of our people who are on a green card started off on an H-1B, said Francois, who is Belgian. Including me.

The Trump administration is considering changes to the scope of the H-1B program, one of the main routes through which U.S. employers sponsor skilled staff for immigration, White House spokesman Sean Spicer told reporters last week.

The administration may push to require companies to try to hire Americans first, and make it more difficult for lower-paid roles to qualify for the visas, in order to serve, first and foremost, the U.S. national interest, according to a draft executive order seen by Bloomberg. So far, no such changes have been announced, and the White House hasnt confirmed any details. Broader changes might require an act of Congress.

Politicians in both parties, as well as labor unions, have criticized the H-1B program because they say it lets companies undercut American wages. Some Democrats have also sought to restrict H-1B visas, including through a bill introduced by U.S. Representative Zoe Lofgren of California that would raise the minimum pay for such visa holders to $130,000.

Read More: Why H-1B Tech-Worker Visas Press Trumps Buttons

India-based outsourcing firms, such as Infosys Ltd. and Wipro Ltd., are among their largest users. Silicon Valley giants including Google Inc. and Facebook Inc. are also major H-1B sponsors, and have lobbied for years to make the category more flexible.

The Trump administration didnt respond to requests for comment. Trump has said its necessary to restrict travel from the seven countries because of the risk of terrorism.

U.S. bioscience firms employ about 1.7 million people, according to an industry-backed study, including a rising number of foreigners. Fewer than half of biomedical scientists in the U.S. in 2014 were native-born citizens, according to the journal Nature, and a third were non-citizens. Meanwhile, thousands of foreign scientists travel to the U.S. every year for specific projects.

The chief executives of two companies with U.S. operations, Roche Holding AG, Europes largest drug-maker, and Londons AstraZeneca Plc,assailed the restrictions.

Science doesnt have any borders, so anything that gets in the way of a borderless science exchange doesnt help, said Pascal Soriot,CEO of AstraZeneca, which has U.S. headquarters in Wilmington, Delaware, and research and manufacturing sites in Massachusetts and Maryland. We want to be able to move our people and our scientists around the world.

Trumps first executive order on immigration gave companies an early hint of what could become broader struggles. At Monrovia, California-based Xencor Inc., which works on drugs for immune diseases and cancer, one employee had to cancel a trip for fear of being stranded and another abandoned hope of a visit from an elderly relative overseas, according to CEO Bassil Dahiyat.

Xencor relies on its non-American staff, Dahiyat said: I can think of several off the top of my head who are absolutely pivotal.

Olivier Elemento, a computational biologist at Weill Cornell Medical College in New York, runs a cancer systems lab where two researchers have H-1B visas. The lab works on computational biology, where researchers analyze complex patient gene data to help find targets for new precision drugs -- an approach that has led to recent breakthroughs.

Our research would suffer without a doubt, Elemento said. A lot of discoveries we would make maybe wouldnt happen.

The immigration battle marks the second between drugmakers and the president. Last month, Trump accused the companies of getting away with murder by charging high prices for medicine and threatened to use the purchasing muscle of the federal government to drive costs down.

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On Tuesday, more than 160 industry executives, scientists, and investors signed a letter to the journal Nature Biotechnology condemning Trumps earlier travel ban as a misguided threat to both liberty and innovation.

Medical leaders point to the central role of foreign doctors in the kinds of crises Americans fear. When the Boston Marathon bombings tore the city apart almost four years ago, a Greek doctor led Massachusetts Generals response: George Velmahos, chief of trauma surgery.

Mass General and its affiliated Brigham and Womens Hospital have 100 people with visas from the seven countries targeted by Trumps order, according to Katrina Armstrong, Massachusetts Generals physician-in-chief.

We brought them here because we thought they had the talent to make the country a better place, Armstrong said. These are people who couldnt be more dedicated to preventing violence and saving lives.

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Trump's H-1B Visa Crackdown Threatens Cutting-Edge US Medicine - Bloomberg