Next, we compared the sequences of the new strain with the related H5N1 from avian flu and H1N1 from the Spanish flu ( Figure 5 ). Among 387 residues that were structurally modelled, the "2009 H1N1" neuraminidase differs from the other two in 21 positions. The mapping to the structure ( Figure 4B ) shows that the novel sequence mutations are distributed all around the surface of the molecule leaving the hydrophobic core, but also the catalytic site, essentially untouched. Importantly, none of the new mutations appears sufficiently close to affect the drug binding pocket. For example, all 17 residues within 3 Å of the zanamivir molecule bound to the active site are fully conserved among all three strains. The closest mutation is the conservative V149I substitution at a distance of ~10 Å to zanamivir and ~7 Å to oseltavimir.