TMPRSS2 and infection: In cells with high transmembrane protease serines 2 (TMPRSS2) expression (e.g. Calu-3, a human Lung Cancer Cell Line which is with high TMPRSS2 expression but deficiency in cathepsin L), SARS-CoV-2 enter into cells by membrane fusion-mediated infection after TMPRSS2 cleaves S protein into S1 and S2 subunits (54, 55), and binds with S2 subunit (55, 56).