What are the remaining important questions regarding receptor recognition  mechanisms  of coronaviruses? First, what are the crystal structures of α-coronavirus S1-NTDs, γ-coronavirus S1-NTDs, and γ-coronavirus S1-CTDs? We have hypothesized  that α-coronavirus and γ-coronavirus S1-NTDs have a galectine fold and that γ-coronavirus  S1-CTDs have either a β-sandwich fold or β-sheet fold. These hypotheses need to be tested using experimentally determined crystal structures of these S1 domains.