Furin cut protein chain carries signature target cutting site.This sequence amino acid units proline-arginine-arginine-alanine , PRRA code refers amino acid letter alphabet.PRRA amino acid sequence core SARS2 ’ furin cleavage site.Viruses kinds clever tricks , furin cleavage site stand ? Because known SARS-related beta-coronaviruses , SARS2 possesses furin cleavage site.All viruses S2 unit cleaved different site different mechanism.How SARS2 acquire furin cleavage site ? Either site evolved naturally , inserted researchers S1/S2 junction gain-of-function experiment.Consider natural origin first.Two ways viruses evolve mutation recombination.Mutation process random change DNA ( RNA coronaviruses ) usually results one amino acid protein chain switched another .