@swcrisis The importance of the FCS has been known for many years and Chinese scientists were toying with it.
Here is a GoF work from a Beijing team, published in Oct 2019, with point mutation of a chicken coronavirus to get a working FCS.
@swcrisis They changed PRGR in rYN just before the S2 subsite to RRKR, to make it an FCS (R-X-KorR-R is the canonical FCS motif).
This increased the viral load by x10 after 36 hours. Survival of 1-day-old chickens went from 90% to 10%.
@swcrisis According to the paper, they were inspired by similar work on SARS-CoV
@swcrisis Here is a good summary of the FCS motif, across many organisms:
@swcrisis By the way the reference for the creation of a SARS-CoV FCS with motif RRKR in the Beijing paper from Oct 2019 is wrong.
 points to something different.
The likely reference is Watanabe et al, Dec 2008: