FURIN and influenza: FURIN is a well-conserved member of the PCSK family that is expressed ubiquitously in all vertebrates.7 More than a hundred furin target proteins have been identified in the mammalian proteome,8 and many bacterial toxins and viruses such as cholera and diphtheria toxins, influenza and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) depend on furin-mediated cleavage for their pathogenicity.9 Furin-inhibiting antibodies and small-molecule drugs have been shown to protect against anthrax toxin10 and to prevent viral glycoprotein maturation in vivo.