HCoV-OC43 does not bind to and agglutinate erythrocytes pretreated with 9-O-acetyl esterase from either influenza C virus or bovine CoV [190]. HCoV-HKU1 does not infect primary human ciliated airway epithelial cells pretreated with an expressed HKU1 hemagglutinin-esterase (HE) protein possessing 9-O-acetylesterase activity [191]. These findings suggest that both HCoV-OC43 and HCoV-HKU1 bind to 9-O-acetylated sialyl glycans (Figure 4a) on the host cell surface for mediating virus infection. As shown in Table 2, the 9-O-Ac-Sia receptor-binding function of homodimeric HE proteins, comprised of a receptor-binding (lectin) domain and receptor-destroying domain, of HCoV-OC43 and HCoV-HKU1 was reported to be lost, and its loss was reported to be associated with an accumulation of mutations in the OC43-HE lectin domain or massive deletions found in the HKU1-HE lectin domain during evolution in humans [94]. Binding of the S1 subunit of another type of spike, a homotrimeric spike (S) protein (Figure 2), of HCoV-OC43 and HCoV-HKU1 on human rhabdomyosarcoma cells was shown and was reported to be reduced by pretreating the cells with HKU1-HE, OC43-HE or BCoV-HE, but not by pretreating the cells with MHV-S-HE, possessing 4-O-acetylesterase activity [191]. These findings suggested that 9-O-Ac-Sia is an essential receptor for infection of HCoV-OC43 and HCoV-HKU1 mediated by the S1 subunit of their S proteins.