O-75 - The furin site is a 12-nt PRRA insert with a rare doubled CGG-CGG arginine codon and a FauI site
The SARS-CoV-2 furin cleavage site is created by a 12-nucleotide insert (TCCTCGGCGGGC) coding PRRA at the S1/S2 junction; the two adjacent arginines are encoded by CGG-CGG, a rare codon in these viruses (only ~5% of arginines use CGG, and this is the only doubled CGG in the genome), and the insert contains a FauI restriction site.
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Why this is evidence
The furin insert’s rare doubled CGG-CGG arginine codon (CGG is expression-host-preferred and rare in these viruses) and embedded FauI site are more expected from lab insertion than from natural acquisition — favouring H-43 over the natural members.