TMPRSS2 and COVID-19: In pulmonary cells that have lost the ability to express cathepsin L at high levels, the S protein is cleaved at these furin-like S1/S2 cleavage sites before internalization, enhancing the effect of TMPRSS2 and internalization of the virus into cells [18], which may partially explain why COVID-19 has greater transmissibility and/or pathogenicity [19, 20].