FURIN and infection: At the same time, the multibasic S1/S2 furin cleavage site (681-PRRAR) as well as the S2′ cleavage site (at 815R) were maintained, rendering maVie16 Spike accessible for proteolytic processing by endogenous proteases such as TMPRSS2 or furin, which are essential for productive infection (Takeda, 2022) and highly conserved between mice and men (Vaarala et al., 2001).