Thx for this discussion, SD. When I saw, I think in Feb. 2020, that SARS CoV2 was using furins to get into human cells, rather than the much less efficient S1/S2 cleavage mechanisms of SARS & MERS, my heart hit the floor as I knew how viciously this bug was going to spread.

As I had also (recently) read that there are no (other) known betacoronaviruses that use furin cleavage, I really appreciate your elucidation on how SARS-CoV2 could have acquired this tool. I needed this information, especially as the O-linked glycans imply a natural, rather than engineered, acquisition.