Abstract The influenza virus M1 (matrix) protein forms the connection between the membrane component of the virus and its replication component eight ribonucleoprotein particles (RNPs). For this activity, M1 self-polymerises in the infected cell in order to pull glycoprotein containing membrane segments together. Later in the process of infection, M1 enters the nucleus and is active in the nuclear export process of newly made RNPs for virus particle assembly. The N-terminal domain (residues 1–164) of M1 carries the nuclear localisation sequence (NLS) motif and is important for membrane binding, self-polymerisation and nuclear export of RNPs. An NLS-mutant M1 has been used in functional studies in order to implicate the positive charges in the NLS in these three activities. In this paper, the crystal structure of the N-terminal domain of this NLS-mutant is determined and is found to be the same as that of the wild-type protein, clearly indicating that it is the absence of the positively charged residues of the NLS that causes the knock-out phenotype rather than a change in the overall structure of the mutant protein.