These number and position of the clusters indicated by the red ellipses differ between Figure S3 (HA sequences) and Figure S4 (NA sequences) probably because there are different selection pressures acting on these two genes as they have quite different functions (i.e. the HA protein is used by the virus to bind to the host cell for entry, whereas the NA protein is an enzyme that enables new progeny viruses to leave the host cell). Also, in Figures 3 and 4 , for both the HA and NA gene sequences, respectively, there is a large region of transition between Mos/10/99-like and Fuj/411/02-like viruses, containing sequences collected during 1999-2005.