Activation of VSV-SARS-CoV-2-Atto 565 by brief treatment with trypsin for 30 minutes followed by the addition of the trypsin inhibitor aprotinin bypassed the requirement for host cell proteases TMPRSS2 and cathepsin L for infection--implicating that trypsin treatment cleaved the S2’ site (52, 53). Here, CTSL is linked to infection.