CTSL and COVID-19: The endosomal cysteine proteases CTSL and cathepsin B (CTSB) mediate cleavage of the SARS-CoV-1 S protein, which is necessary for entry of coronavirus into host cells.17,24 Therefore, plasma levels of CTSL, CTSB, ACE2, and its products (angiotensin 1–7 (Ang(1–7)), were measured in patients with COVID-19.