Some avian H5 and H7 subtype viruses acquire a gene mutation that adds one or more basic amino acids to the cleavage site, allowing HA to be activated by ubiquitous proteases.

• 某些禽类的H5及H7亚型病毒获得了某种基因突变，在切割的位置加上了一个或数个碱性胺基酸，使得HA蛋白能由随处都有的蛋白酶活化。