SARS-CoV infection was blocked by specific inhibitors of the pH-sensitive endosomal protease cathepsin L. S-mediated entry into Vero E6 and 293T/hACE2 cells was blocked by leupeptin, Z-lll-FMK (an inhibitor of both CTSB and CTSL), and E64c (an inhibitor of cysteine proteases) [21]. Here, CTSB is linked to severe acute respiratory syndrome.