It sits middle SARS2 spike protein.It also lies heart puzzle virus came from.The spike protein two sub-units different roles.The first , called S1 , recognizes virus ’ target , protein called angiotensin converting enzyme-2 ( ACE2 ) studs surface cells lining human airways.The second , S2 , helps virus , anchored cell , fuse cell ’ membrane.After virus ’ outer membrane coalesced stricken cell , viral genome injected cell , hijacks protein-making machinery forces generate new viruses.But invasion begin S1 S2 subunits cut apart.And , right S1/S2 junction , furin cleavage site ensures spike protein cleaved exactly right place.The virus , model economic design , carry cleaver.It relies cell cleaving it.Human cells protein cutting tool surface known furin .