Now functional reason SARS2 furin cleavage site , cousin viruses ’ , seen lining ( computer ) string nearly 30,000 nucleotides genome cousin coronaviruses , closest far known one called RaTG13.Compared RaTG13 , SARS2 12-nucleotide insert right S1/S2 junction.The insert sequence T-CCT-CGG-CGG-GC.The CCT codes proline , two CGG ’ two arginines , GC beginning GCA codon codes alanine.There several curious features insert oddest two side-by-side CGG codons.Only 5 percent SARS2 ’ arginine codons CGG , double codon CGG-CGG found beta-coronavirus.So SARS2 acquire pair arginine codons favored human cells coronaviruses ? Proponents natural emergence up-hill task explain features SARS2 ’ furin cleavage site.They postulate recombination event site virus ’ genome recombinations rare , insertion 12-nucleotide sequence double arginine codon unknown beta-coronavirus repertoire , site genome would significantly expand virus ’ infectivity .