The spike or S glycoprotein is a transmembrane protein with a molecular weight of about 150 kDa found in the outer portion of the SARS-CoV-2 virus. This protein is composed of two subunits, S1 and S2 and plays a key role in the receptor recognition and cell membrane fusion process. The S1 subunit contains a receptor-binding domain that recognizes and binds to the host receptor angiotensin-converting enzyme 2, while the S2 subunit mediates viral cell membrane fusion.