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People protest against the US immigration ban at John F. Kennedy International Airport in New York City on 28 January.
Kaveh Daneshvar was thrilled when he was invited to speak at a molecular biology meeting next month in Banff, Canada. Daneshvar, a molecular geneticist, is finishing a postdoc at Harvard Medical School and Massachusetts General Hospital in Boston, and is preparing to go on the job market. He hoped that the conference talk would give him much-needed exposure to leaders in his field.

Trump agenda threatens US legacy of science diplomacy
But that now seems impossible: if Daneshvar, an Iranian citizen, leaves the country, he may not be able to return. On 27 January, US President Donald Trump signed a sweeping executive order that blocks refugees from entering the United States for 120 days and stops Syrian refugees indefinitely. It also bans citizens of seven majority-Muslim countries “compromised by terrorism” — Iran, Iraq, Libya, Somalia, Sudan, Syria and Yemen — from entering the United States for 90 days. The US government has issued conflicting statements on whether the provisions apply to people such as Daneshvar who hold visas that would otherwise permit them to live, work or study in the United States — including those with the permanent resident visas known as green cards.
Nature spoke to more than 20 researchers affected by the new policy, who described their feelings of fear, shock and determination. Some asked to remain anonymous for fear of retaliation by the US government.
“I am really appreciative of what the US has given me and allowed me to achieve here, but at the same time this is really shocking,” says Ali Shourideh, an economist at Carnegie Mellon University in Pittsburgh, Pennsylvania. “I've always been under the assumption this is a free country, that once you immigrated they won't try to kick you out or make life hard for you.”
Shourideh, an Iranian citizen with a green card, has travelled to Iran several times recently to visit his mother, who has cancer. Now, if he leaves the United States, he may not be able to return. “You have to make a choice: do I want to see my mom or do I want to keep my job?” he says. “This is something that for sure will hurt us personally, but also the US, I think, because all these high-skilled-type professionals would not want to be here anymore.”