We mapped the level of residue conservation (calculated with the evolutionary trace algorithm [37] ) from the multiple alignment of all NA subtypes to its corresponding position in the structure. The results show the strict conservation close to the neuraminidase catalytic site, which also serves as the drug binding pocket ( Figure 4A ). The remaining conserved patches (for example the sites around N104 or below N146) fit into each other and form the dimerization/tetramerization interfaces [35] . A model of the dimeric version is available in PDB format at http:/ /mendel.bii.a-star.edu.sg/SEQUENCES/H1N1/.