DPP4 and infection: For entering into the host cell and triggering infection, several human proteins assume the role of viral receptors: angiotensin-converting enzyme 2 (ACE2) for SARS-CoV (Li et al., 2003), SARS-CoV-2 (Hoffmann et al., 2020), and HCoV-NL63 (Hofmann et al., 2005); aminopeptidase N (APN) and dipeptidyl peptidase 4 (DPP4) are entry receptors for HCoV-229E and MERS-CoV, respectively; the mice-originated beta-CoVs HCoV-OC43 and HCoV-HKU1 need 9-O-acetylated sialic acid (9-OASA) as a viral receptor (Cui et al., 2019).