There are 9 serotypes of neuraminidase (NA) from influenza A virus (N1 to N9), which are classified into two groups based on primary sequences (groups 1 and 2). The structural hallmark of the two groups is the presence or absence of an extra 150-cavity (formed by the 150-loop) in the active site. Thus far, structures of NAs from 6 out of the 9 serotypes have been solved. Here, we solved the N5 structure, the last unknown structure group 1 serotype with a unique Asn147 residue in its 150-loop, demonstrating that it has an extended 150-cavity that closes upon inhibitor binding.