PROS1 and infection: Although there is no systematic report on the effect of Y756 mutation on the infectivity of infectious viruses at present, refer to the series of studies in which D614G was proved to be an infection-enhancing mutation in tissue culture using a pseudotyped virus system (24, 25, 30, 43, 53); the Y756 mutation in S protein may be infection attenuating, but this needs further confirmation with infectious virus.