PROS1 and infection: S1 subunit S protein receptor‐binding domain (RBD) binds with an ACE2 receptor in the target cell, with two more essential domains: heptad‐repeat 1 (HR1) and the 2 (HR2) domains in the S2 subunit S protein interact to form a six‐HB fusion center to close the virus and cell membranes, thus raising the likelihood of fusion or infection.