Q: What is a furin cleavage site (FCS)?

A: The FCS is a unique feature of SARS-CoV-2 – a sarbecovirus – that makes it more deadly and more contagious than closely related coronaviruses. The FCS of SARS-CoV-2 is located in its “spike” protein, which is a protein found on the surface of SARS-CoV-2, that the virus uses to attach to a receptor on target cells. Furin is an enzyme present in many human tissues that cuts the spike protein at the FCS, making it easier for SARS-Cov-2 to infect cells. SARS-CoV-2 is the only known sarbecovirus with a FCS. The FCS is highly unlikely to have evolved naturally, raising the possibility that it was added to the virus in a laboratory.

Q: What does “Inconsistent with Evolution” refer to?

A: One of the first scientists to note the presence of a FCS in SARS-CoV-2 described this feature of the virus as “inconsistent with expectations from evolutionary theory.” Days later, and in the absence of any convincing scientific reason, this scientist turned into one of the most vocal proponents of the natural origin hypothesis.