Homotetrameric NA is a type II membrane protein, whereas HA is a type I membrane protein. The nascent NA consists of four parts: the cytoplasm tail (in amino-terminus), the transmembrane domain, the stalk domain and the global domain [22] . Different subtypes of NA are composed of 450,480 amino acids, displaying low sequence similarity. Although there is variable homology among the various NA subtype sequences, especially in the N1 and N2 subtypes with the deletion of 4,30 amino acids in the stalk domain [23] , NA subtypes display stable topologies: a six-bladed b-propeller fold makes an enzymatic activity domain that functions in the release of progeny virions [24, 25] .