S:D614G and S:H655Y are gateway mutations that act epistatically to promote SARS-CoV-2 variant fitness.

S:D614G is ubiquitous among SARS-CoV-2 B-lineage Spikes and is required for infectivity of the main Variants of ConcernIn an example of convergent evolution, rare SARS-CoV-2 A lineage viruses maintained transmission chains in the absence of S:D614G, but were instead dependent upon S:Q613H or S:H655YS:D614G and S:H655Y are both adaptations to the polybasic S1/S2 cleavage siteIncreased infectivity of S:D614G and S:H655Y is associated with a more open RBD conformation and increased steady-state levels of virion-associated S1.