PROS1 and COVID-19: Currently, there are two common strategies exist in the design of the COVID-19 S protein: the 2P mutation (which substitutes two amino acids in the S2 subunit with proline to stabilize the prefusion conformation of the S protein) and alteration to the S1/S2 cleavage site mutation (which deletes or alters the cleavage site sequence to prevent S protein cleavage in the host cell, thereby maintaining its structural stability).