The website Rootclaim, which uses Bayesian analysis to estimate the probabilities of various hypotheses about news events, claims to have ascertained that with high (approximately 75% probability), SARS-CoV-2 was produced by gain-of-function research in a lab (most likely the Wuhan Institute of Virology (WIV)) rather than through transfer from animals to humans, as is commonly believed:


Two of the key pieces of evidence that the site claims point to a lab origin are:

(1) SARS-CoV-2 is a "chimera" - it contains genetic code from two other viruses - and these other viruses are likely to have been available to the WIV to combine but are not likely to have been combined in nature.

(2) SARS-CoV-2 contains a "furin cleavage site" - a feature making it more contagious - and the fact that the cleavage site arose via insertion rather than mutation, as well as the fact that it uses the CGG codon for arginine, are both features that are likely to have occurred if the virus was engineered but are less likely to occur if the virus was natural.

Are these claims accurate? Is it true that these features point toward a lab origin? Are there other features that point away from a lab origin that have not been analyzed by this site, or is the analysis as a whole accurate?

  • Does this answer your question? Was the virus which causes COVID-19 made in a Chinese lab? – PC Luddite Apr 4 at 4:13
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    @PCLuddite This claim is different than the linked question. Still dubious, but different. – David Hammen Apr 4 at 17:48
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    @DavidHammen maybe there's a better one to link as a dupe. Just feel like we've had a lot of these "made in a lab" questions. Most were probably deleted for being much lower quality though. – PC Luddite Apr 5 at 3:05

According to an article by ShanghaiTech University Furin cleavage sites naturally occur in coronaviruses Stem Cell Research Volume 50, January 2021:

SARS-CoV-2 has the furin recognition motif at S1/S2, causing by a 12-nucleotide insertion not presented even in its closest relatives (Walls et al., 2020, Coutard et al., 2020). This stimulates a conspiracy that this furin site can only be manual work, thus SARS-CoV-2 must be created in a laboratory. Here, we analyzed the sequences of coronaviruses and found furin sites occurred independently for multiple times during evolution. This exhibits natural occurrence of furin cleavage site in SARS-CoV-2 spike protein is highly possible. Thus, the insertion of furin cleavage site into SARS-CoV-2 spike protein is not necessarily a result of manual work.


Furin cleavage sites in spike proteins naturally occurred independently for multiple times in coronaviruses. Such feature of SARS-CoV-2 spike protein is not necessarily a product of manual intervention, though our observation does not rule out the lab-engineered scenario.

  • I'm not a biologist but I believe the claim is specifically about sarbecoviruses, not all coronaviruses. There's also a claim about the presence of a rare restriction site (CGG) around the furin cleavage site. – Avery Apr 5 at 14:59
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    The question asks if the furin cleavage site is evidence in favor of lab leak hypotheses, but this paper states that it "is not necessarily a product of", which seems to be saying that it isn't proof, but proof if a very different standard than "is evidence in favor of". – Jayson Virissimo Apr 5 at 17:17

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