FURIN and avian influenza: This is well documented for the hemagglutinin protein of highly pathogenic avian influenza strains.26 In the case of coronaviruses, the cleavage site changes in HKU205 are reminiscent of the mutations selected in the spike S1/S2 and S2′ cleavage sites of feline coronaviruses that are macrophage-tropic and the cause of lethal peritonitis in cats.27, 28 For MERS-CoV, we have shown previously that the S2′ furin cleavage site is not optimal,8 and so may be only a few mutational steps away from acquiring a more efficiently processed polybasic furin site.