The HA is synthesized as precursor protein and cleaved by cellular serine proteases into the functional proteins HA1 and HA2. The amino acid sequence at the cleavage site determines HA processing by cellular proteases and thus, also the organ tropism. Mutations at the cleavage site of avian influenza viruses may lead to an insertion of alkaline amino acids. Ubiquitous proteases such as furin can cleave such a mutated HA and, as a consequence, permit systemic spreading of the virus. This mechanism changes a lowly pathogenic into a highly pathogenic avian influenza virus and has been associated so far only with subtype H5 and H7 viruses.The polymerase complexes consisting of the three polymerase proteins PB1, PB2, and PA are located at the ends of the nucleocapsids. These helical capsids are encircled by the M1 matrix protein and by a host-derived lipid bilayer envelope in which the virus surface glycoproteins haemagglutinin (HA) and neuraminidase (NA) as well as the M2 matrix protein are embedded.