For examples, the S protein of CoV-229E and transmissible gastroenteritis virus (TGEV) employ CD13 (aminopeptidase N) as a receptor for entry and infection of target cells [68,69], whereas S protein of CoV-NL63 and HKU1 interact with glycan-based receptors carrying 9-O-acetylated sialic acid (9-O-Ac-Sia) [70,71]. Here, PROS1 is linked to infection.