In β-CoV, HE mediates viral attachment to O-Ac-SAs and its function relies on the combined CBD and RDE domains. Most β-CoVs target 9-O-Ac-SAs (type I), but certain strains switched to alternatively targeting 4-O-Ac-SAs (type II). For example, the SA-acetylesterase enzyme in BCoVs and HCoV-OC43 is known to have hemagglutinizing activities as a type of SA-9-O-acetylesterase [8]. The SA-acetylesterase is the HE surface glycoprotein in BCoV. The three-dimensional structure of BCoV HE is similar to other viral esterases [9]. The HE gene is found only in the β-CoV genus. The acetylesterase of murine CoVs differs in its substrate binding specificity from that of BCoV and HCoV-OC43, which is specific for O-acetyl residue release from SA C-9. Murine CoVs prefer to esterize 4-O-acetyl-NeuAc [64]. The β-CoV acetylesterase destroys the receptors and this specificity is similar to that of influenza viruses. Acetylesterase activity can be inhibited by diisopropyl fluorophosphate and this agent decreases viral infection levels [65]. As deduced from the SA acetylesterase of HCoV-OC43 [8], the 9-O-Ac-SA species is a receptor binding determinant for erythrocytes and entry into cells [59]. The BCoV HE protein has dual activity of acetylesterase and HA [9]. BCoV widely agglutinates erythrocytes and purified HE only agglutinates Neu5,9Ac2-enriched erythrocytes of rats and mice. BCoV and HCoV-OC43 can agglutinate chicken erythrocytes, while purified HE cannot. In contrast to the HE protein, purified S glycoprotein can agglutinate chicken erythrocytes [52], indicating that the major HA is the S protein which acts as the major SA-binding protein. However, the role of O-Ac-SAs is not certain to be essential in receptors, and SA-binding activity may be essential only to the HE protein, but not to the S glycoprotein [54].