ACE2 and viral infectious disease: Taking into consideration the significant impact of PD-L1 glycosylation on cancer therapy and clinical diagnosis, it is worth mentioning that the transmembrane spike protein of the novel coronavirus causing the current coronavirus disease 2019 (COVID-19) pandemic is also highly glycosylated, essential for binding to a human cellular receptor angiotensin converting enzymes 2 (ACE2), and required for viral infection [118–120].