- NIH virologist Vincent Munster emailed his postdoc on Oct. 6, 2019, proposing to engineer a furin cleavage site into a coronavirus spike protein to "enhance infectivity."
- The proposed modification matches the central genetic feature cited by lab-leak proponents as evidence SARS-CoV-2 did not arise naturally.
- Munster's lab also solicited and received unpublished bat coronavirus spike sequences from the U.S.-funded PREDICT program months before the pandemic.
- The furin site Munster suggested differs from the one found in SARS-CoV-2, which has a "suboptimal" motif researchers say would be illogical for an engineered virus.
- Munster was indicted in 2026 for smuggling mpox virus samples into the U.S. without proper documentation.
The proposal: Engineering a furin cleavage site
One Sunday morning in October 2019, Vincent Munster, chief of the virus ecology unit at the National Institutes of Health's Rocky Mountain Laboratories in Hamilton, Montana, emailed a postdoctoral scientist in his lab with an idea. Eleven weeks before a coronavirus carrying a similar feature emerged in Wuhan, China, Munster proposed engineering a furin cleavage site into a SARS-like spike protein to make it more infectious.
The email exchange — obtained by U.S. Right to Know through a Freedom of Information Act request and never before made public — shows Munster suggesting his postdoc Michael Letko "make S with a more broader cleavage signature (like furin-like) to enhance infectivity." In virology parlance, "S" refers to the coronavirus spike protein that allows the virus to latch onto cells. Munster even provided the genetic motif: "R-X-[K/R]-R."
Scientific context behind the furin site
The furin cleavage site found in SARS-CoV-2 remains the only known such feature among all SARS-related coronaviruses found in nature, making it the single genetic characteristic most frequently cited by proponents of a laboratory origin for the pandemic.
Munster's suggestion followed his reading of a preprint from virologists Vineet Menachery and Ralph Baric showing that the protease trypsin could unlock entry for coronaviruses that otherwise would not infect human cells. Letko had forwarded the paper two days earlier, noting "Seems like more groups will be converging on this idea soon."
However, the furin cleavage site Munster proposed differs from the one found in SARS-CoV-2. The pandemic virus carries a "suboptimal" RRAR motif, while Munster suggested the textbook canonical form R-X-K/R-R. This discrepancy, the records show, aligns with arguments from scientists who contend an engineered virus would have used the optimal sequence.
Unpublished sequences: A second thread
Beyond the furin site proposal, the records reveal Munster's lab actively solicited unpublished bat coronavirus spike sequences from the U.S.-funded PREDICT consortium, whose lead partner EcoHealth Alliance had long-standing collaborations with the Wuhan Institute of Virology.
In March 2019, Munster emailed EcoHealth President Peter Daszak and Columbia University virologist Simon Anthony seeking "additional, unpublished lineage B beta coronavirus sequences." The request aimed to test which bat coronaviruses could infect human cells using Letko's newly developed "rapid and scalable platform."
Within two months, Anthony shared sequences from a Malaysian bat virus and one from Uganda and Rwanda. Neither was closely related enough to SARS-CoV-2 to have given rise to it. But the exchange demonstrated that U.S. government scientists were using unpublished viral sequences precisely because they were unpublished — contradicting a 2021 Cell review that called such scenarios implausible.
Historical parallels to DEFUSE proposal
The Munster email is the second known instance of scientists proposing to insert a furin cleavage site into a SARS-like spike within two years of COVID-19's emergence. In 2018, EcoHealth Alliance, Baric's lab and Wuhan Institute of Virology researchers had proposed similar modifications to the Defense Advanced Research Projects Agency under a program called DEFUSE. That proposal was denied over gain-of-function research concerns.
Sen. Rand Paul has alleged that Munster was listed as a partner in the DEFUSE proposal presented at a January 2018 DARPA event, though Munster was removed from the final submission. The newly released records show no direct link between Munster and DEFUSE.
What happened next
The records show no evidence that Letko followed through on Munster's suggestion to engineer a furin cleavage site. When Chinese scientists posted the novel coronavirus sequence on Jan. 10, 2020, Munster's lab pivoted to testing the actual pandemic virus. Their paper, published in Nature within 12 days, confirmed that SARS-CoV-2 used the ACE2 receptor for cell entry.
The paper's second figure — titled "Trypsin enhances lineage B entry in various cell lines" — used trypsin rather than the furin Munster had proposed. Neither of the PREDICT sequences appeared in the final publication.
Munster did not respond to requests for comment. He was indicted earlier this year for bringing mpox specimens into the U.S. without proper documentation and making false statements to law enforcement, charges to which he has pleaded not guilty.
Implications for origins debate
The newly disclosed emails do not prove SARS-CoV-2 was engineered in a laboratory. Nothing links Munster's research to the pandemic virus, and the furin site he proposed differs from the one found in nature. But the records establish that weeks before COVID-19 emerged, a senior NIH virologist proposed in writing to build the very feature that has become the central argument for a laboratory origin.
As molecular biologist Alina Chan, who reviewed the records, noted: "What this shows is that different groups of virologists were converging on the idea of inserting cleavage sites in the spikes of potentially dangerous coronaviruses. Assuming a lab origin of COVID-19, if a lab in Wuhan hadn't caused the pandemic, it's plausible that a lab in the U.S. would've eventually followed the same path."
The records also raise questions about transparency. The NIH press office declined to answer why the agency had not previously disclosed that its scientists proposed engineering a furin site, or that they solicited and obtained unpublished SARS-like coronavirus sequences, before the pandemic began.
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