The first published structural characterization of SARS-CoV-2’s spike S1/S2 furin cleavage site: notes the 12-nucleotide PRRA-encoding insertion is absent from every other lineage-B (sarbecovirus) genome sequenced at the time, including SARS-CoV-1. Proposes the site’s furin/TMPRSS2-mediated processing may explain expanded tissue and species tropism relative to SARS-CoV-1. relevance_note: establishes the basic empirical fact — an FCS unique among close relatives at time of writing — on which all later engineering-signal arguments (both directions) are built.