The spike (S) protein, which is projected outward on the surface of the virion is the very important pathogenic determinant. The S protein has two subunits, S1 and S2. The S1 binds on angiotensin-converting enzyme 2 (ACE2), the host cell receptor, and the S2 mediates membrane fusion. The receptor-binding domain (RBD) of S1 binds ACE2. The Cryo-electron microscopic structure of the spike protein of SARS-CoV-2 has been determined at 3.5°A [15]. The study showed SARS-CoV-2 S protein has higher affinity to ACE2 compared to that of SARS-CoV. The mutation at 614 position in the S protein of SARS-CoV-2 from aspartic acid to glycine (D614G) was identified to reduce S1 shedding and increases infectivity [17]. The sequence analysis showed that the G614 genotype has steadily increased from March to May 2020, indicating a transmission advantage of the virus with G614 mutation [17].