ACE2 and Respiratory tract infection: Whereas all sHCoV cause respiratory tract infections, HCoV-OC43, HCoV-229E, HCoV-NL63 and HCoV-HKU1 are genetically dissimilar (Fig. 1B), belonging to two distinct taxonomic genera (Alpha and Beta), and use different receptors: HCoV-229E and HCoV-NL63 have adopted cell surface enzymes as receptors, aminopeptidase-N (APN) for HCoV-229E and angiotensin-converting enzyme 2 (ACE2) for HCoV-NL63, while HCoV-OC43 and HCoV-HKU1 use 9-O-acetylated sialic acid [18, 32].