It started with a tweet.

Jürgen Kleine-Vehn, a plant scientist based in Austria, had just read about the story of Samira Asgari, one of many Iranian scientists affected by President Trump’s executive order banning citizens of seven predominantly Muslim countries from entering the U.S. On Saturday, Asgari was headed to Boston to start a postdoctoral fellowship on tuberculosis, but was prevented from boarding a plane in Frankfurt. Hundreds, perhaps thousands, of scientists have been similarly affected, as I reported on Sunday.

“I cannot believe what is happening,” Kleine-Vehn tweeted.

Magnus Nordborg, another Austrian-based plant scientist, replied: “I think we can host people who are stuck, right? Preferably in a suitable lab, of course.”

Others were thinking along similar lines. The Gulbenkian Institute in Portugal had pledged to “provide what aid we can to scientists trapped by [the immigration ban],” and the Brain and Spine Institute in Belgium said it was “ready to welcome all our colleagues who are in France, in Europe, and who need a place of reception during this difficult period.”

After talking to Nordborg, Maria Leptin, the director of the European Molecular Biology Organization, decided to collate the offers of help. “I felt that words are one thing, but we need deeds,” she says. “I thought: Let’s get people together and do it in a big way.” And EMBO was the perfect organization for it; it’s a prestigious network of European biologists, and one of its goals is specifically to “enable international exchange between scientists.”