Flow cytometry dot plots for wild-type and ACE2 random mutant library clones enriched after three rounds of FACS. X-axes denote Alexa488 fluorescence (ACE2 display). Y-axes denote Alexa647 fluorescence (ACE2 binding to spike protein). Plots depict dots for approximately 3*10⁴ yeast cells. ACE2 yeast incubated with spike as follows: human– 1 nM, cat– 4 nM, pig– 4 nM, dog– 25 nM. For poorly understood biological reasons homogeneous populations of yeast carrying identical display plasmids feature 25% or greater cells (lower left of plots) that do not display protein.