TMPRSS2-mediated cleavage at the S1/S2 and the S2′ sites is indispensable for infectivity in most cells; except for human lung cancer Calu-3 cells [102] that utilize endosomal cysteine proteases cathepsins B/L (Cat B/L) [103] or proprotease furin [104]. Here, CTSB is linked to lung cancer.