6atk-class I (yellow), 6u7e-class III (pale green), 6u7f-class IV (marine) and 6u7e-class V (salmon)) of the S1-CTD RBDs of HCoV-229E variants in complex with human APN (transparent blue). (b) Positions of important aa residues in receptor binding sites (RBSs) of the four classes of the S1-CTD RBDs that have direct interactions with human APN. (c) Alignment of aa sequences of six classes of the S1-CTD RBDs.