The Furin Cleavage Site Part 1: Nexus

The interface between S1 | S2 • US | China • Military | Civilian • SARS | MERS

The FCS has been the standout feature of the SARS-CoV-2 both in its awkward protrusion from a previously conserved part of the spike sequence, and in the controversy and discussion it has raised.

Several theories have been proposed to explain its precise composition:

1. An unconnected sequence from a bat virus – HKU9
2. A sequence from a 2017 Moderna patent
3. Human α-ENaC
4. An FCS identical to a Feline Peritonitis strain
5. A patent for a SARS vaccine

There is interesting background, and some merit to all of these, but they have in common that they seek an external source of the exact nucleotide sequence for the insert. But it isn’t necessary to find an exact match in literature or a database, however appealing its simplicity. It’s likely that the FCS came about through…