Evolution of coronavirus spikes. (a) Structural comparison between human galectins and alphacoronavirus HCoV-NL63 S1-CTD. Both the crystal structures and structural topologies of the two proteins are shown. Common subcore structures in the two proteins are highlighted with gray shading. (b) Hypothesized evolution of coronavirus spike proteins. Abbreviations: HCoV-NL63, human coronavirus NL63; IC, intracellular tail; S1-CTD, S1 C-terminal domain; S1-NTD, S1 N-terminal domain; TM, transmembrane anchor.