Uses ancestral-sequence reconstruction across sarbecoviruses to test whether the 12-nucleotide insertion producing SARS-CoV-2’s furin cleavage site fell cleanly between codons (as template-switching recombination would tend to) or interrupted one. Finds ~97% probability the insertion disrupted a serine codon in the reconstructed ancestral sequence, which the authors argue sits more comfortably with certain insertion/mutational mechanisms than with a clean natural recombination breakpoint. relevance_note: the primary quantitative test of the “out-of-frame insertion” argument named explicitly in this slice’s brief.