Scientists are speaking out against a directive by the Trump administration that has shut down research into a cure for HIV.

A scientist who was supposed to supply mice that have been modified with human fetal tissue for an HIV study emailed researchers that the Department of Health and Human Services (HHS) “has directed me to discontinue procuring fetal tissue.”

“This effectively stops all of our research to discover a cure for HIV,” he wrote.

A “pause” on research that uses aborted fetal tissue, something Christian conservatives strongly oppose, was announced by the Trump administration this past September. The move will affect numerous biomedical research programs.

Congressional Republicans have tried to ban all fetal tissue research for years to appease the religious right. 85 Republican House members wrote a letter to the head of the FDA asking for a ban on fetal tissue research shortly before the “pause” was announced.

The ban has long been a goal of the anti-abortion industry and has been part of the most recent attacks on Planned Parenthood. The group was falsely accused of selling aborted fetuses earlier this year.

In a statement, the NIH said that there is only a “pause in place” on the research, which is “an action NIH thought was prudent given the examination of these procurements.”

“We were all poised to go and then the bombshell was dropped,” said Warner Greene, director of the Gladstone Center for HIV Cure Research. Greene was about to collaborate with the Montana lab on HIV research when the project came to a halt.

“The decision completely knocked our collaboration off the rails. We were devastated.”

Related: Will Trump’s budget cuts to medical research stall a cure for HIV?

The study would have relied on fetal tissue donated by women who have had legal abortions in order to produce “humanized mice,” mice that have human-like immune systems, according to Science. The mice have been used for years for research in infectious diseases, including HIV.

Mice are one of the few non-human animals that can be infected with HIV, which makes them key to testing HIV medication.

Researchers at the lab were going to test a promising antibody they developed to prevent HIV from developing reservoirs, which are cells infected with HIV but are not producing the virus. These cells aren’t affected by currently available HIV medication.

“You spend your life trying to do good experiments and organize your science carefully, and suddenly, at the whim of some politicians in Washington, D.C., they remove a critical piece of your scientific armamentarium,” Greene said.

On September 28, researcher Kim Hasenkrug, who was supposed to provide humanized mice for Greene’s study on the HIV antibody, sent an email to Greene explaining that HHS told them to stop using fetal tissue.

[HHS] has directed me to discontinue procuring fetal tissue from ABR, the only source for us. I think that they are the only provider of fetal tissue for scientists in the nation who don’t have direct access to aborted fetal tissue. This effectively stops all of our research to discover a cure for HIV.

“The mice were ready,” Greene said. “Just as we’d shipped antibody [to Montana], we were ready to go, and boom, the rug was pulled out from underneath us.”

It is unclear whether non-NIH labs that receive NIH funding will also be banned from using fetal tissue. If so, even more research will be affected.

“Everything I am doing involves humanized mice. It would shut my lab down if we were not able to use fetal tissues,” said Jerome Zack, who has used humanized mice for 25 years to study HIV at UCLA.

But Greene said that the NIH’s actions have already slowed down progress on a cure.

“If we were given the green light right now, it would probably take us a year to get back in the position we were in when the ban was put in place,” he said.

Earlier this year, the Department of Health and Human Services announced a new Conscience and Religious Freedom Division that would handle complaints from health care providers who did not want to take part in an abortion or treat transgender people.