“The two hypotheses, ‘natural’ origin and ‘laboratory’ virus”, for the coronavirus are at the moment “both plausible, and anyone who says that either is ‘certain’, or that it is certainly wrong, makes an unlikely statement. justified by current knowledge “. This was specified by the Italian virologist Guido Silvestri, a lecturer in the US at Emory University in Atlanta, who in a long post on Facebook tries to clarify a topic that is “not only quite thorny from a technical point of view, but also prone to being contaminated from political considerations “. “As for me – says the expert – I share word for word what was written in ‘Science’ just a few days ago by a group of virology giants, including Pam Bjorkman, Ralph Baric, David Relman, Ruslan Medzhitov, Michael Worobey : ‘We must take both the natural origin hypothesis and the laboratory origin hypothesis seriously. A valid investigation must be transparent, objective, data-driven, done by experts from various disciplines, subject to independent review, and managed in in such a way as to limit conflicts of interest. Public health agencies and research laboratories must open their archives to the public ‘”. Silvestri states that explaining the elements currently available to science to dispel the doubt about the origin of the pandemic pathogen” does not it’s easy, but I try anyway, because unfortunately on this issue – he underlines – there is so much confusion due to people talking and ranting without understanding anything about virology “. (continues)” In particular are – writes Silvestri on Fb – I will briefly talk about the ‘mysterious’ sequence of 12 nucleotides present in the Sars-CoV-2 genome to which I mentioned “in a television interview aired on Sunday 30 May during the broadcast’ Mezz’ora in more on Rai3. “I refer to the cleavage site of furin (a proteolytic enzyme) which is present between subunit 1 (S1) and subunit 2 (S2) of the Spike (S) of Sars-CoV-2. It is 4 amino acid residues , Prra (proline-arginine-arginine-alanine), corresponding precisely to 12 nucleotides (therefore 4 codons, CCT-CGG-CGG-GCA), inserted in a strategic position between serine 680 and arginine 681 of the Spike protein. taking as reference the Bat virus Rhinolophus affinis RatG13, since it is the virus closest phylogenetically to Sars-CoV-2 “. “According to the hypothesis of natural origin ‘(the famous leap of species from bat to man, with or without an intermediate host) – details the virologist – this sequence of 12 nucleotides could have been inserted as a result of a random mutation (event extremely unlikely for a series of reasons related to the biology of RNA replication in coronaviruses that I cannot elaborate on here), or following a recombination event that could have occurred in an animal infected simultaneously with two different viruses (a very more likely). On the contrary, according to the laboratory virus hypothesis, the sequence would have been artificially inserted as part of an in vitro viral genome manipulation experiment, done to study the pathogenetic mechanisms of human coronaviruses. ” (continues) “If we move within the hypothesis of natural origin, the curious thing about the furin-cleavage site – points out Silvestri – is that the 2 arginines (RR) are synthesized starting from 2 CGG-CGG codons which, as such, have never been identified (at least as far as I know at the moment) in any other pair of arginines that are part of a furin-cleavage site of other known coronaviruses with which RaTG13 would have recombined to form Sars-CoV-2. In other words, the virus that would have provided the specific nucleotide sequence corresponding to Prra is not known at the moment. It should be noted that it is very possible that one day this virus will be identified, because there are coronaviruses in nature very many, but for now we have not discovered this virus “. “If you want – continues the scientist – at this point it is up to each of us to decide, as good followers of William of Occam, which hypothesis is more parsimonious. The one of natural origin postulates the presence of a virus in nature that has the sequence of 12 nucleotides of the cleavage site for furin with the 2 paired CGG-CGG codons, and requires an explanation of how the virus originated from Yunnan province, where bats are thousands of km from Wuhan, to start the epidemic in the fish market of Huanan, which is a stone’s throw from the Wuhan Institute of Virology (where human coronaviruses are studied). However, even the hypothesis of the laboratory virus is not very Occamist, as at the moment there is no ‘there is absolutely no evidence that researchers, in Wuhan or elsewhere, have created and inserted the above 12-nucleotide sequence into RaTG13. ” Hence the conclusion: at the moment neither hypothesis is certain, but both are plausible and must be investigated.