Some keyfindings from implementing this assay have provided insightsrevealing that (a) influenza binding to receptor functions primarilyas an attachment factor,15 (b) in contrast,ACE2 receptor binding conformationally activates SARS-CoV-2 spikeprotein but these changes are also thermally accessible in the absenceof ACE2,19 (c) an off-pathway state existsin Zika fusion that determines pH sensitivity,17 and (d) influenza fusion also likely has an off-pathwaystate, as endosomal lipids modulate the extent of full fusion butdo not affect the observed extent of hemifusion or initial lipid mixing.31 Here, ACE2 is linked to influenza.