The multibasic S1/S2 furin cleavage site in SARS-CoV-2 S is essential for the infection of human lung cells [30], and multiple studies have reported that this site can be rapidly lost during passage in Vero E6 cells [31,32], likely driven by the enhanced growth of furin deletion mutants in Vero E6 cells [33]. Here, FURIN is linked to infection.