TMPRSS2 and infection: In contrast to SARS-CoV and other group 2B coronaviruses, SARS-CoV-2 has a furin cleavage site between the S1 and S2 subunits, which promotes infection of cells expressing the transmembrane serine protease 2 (TMPRSS2) on their surface, e.g., human respiratory tract epithelial cells (11, 12).