Generally, genes usually encode proteins therefore the S gene specifically encodes the spike protein. The spike protein, a homotrimeric glycoprotein complex, is necessary for the spread of infection.
Mammalian ACE2 (angiotensin-converting enzyme 2) is highly specific for the receptor-binding domain (RBD) in S1 of the complex. Angiotensin II, a body-wide vasoactive polypeptide hormone, is regulated by this protein on the cell surface.