During viral infection, the spike protein is cleaved into these S1 and S2 subunits by nearby host proteases, such as human airway trypsin-like protease (HAT), cathepsins and transmembrane protease serine 2 (TMPRSS2), and releases the signal peptide to promote virus entry into host cells [7,89]. This evidence concerns the gene TMPRSS2 and viral infectious disease.