(a) A structure-based sequence alignment of the CTD domains of NS1 from influenza A and influenza B strains. Basic residues of the NS1B CTD that affect RNA binding activity when mutated to Ala are indicated with green dots. (b) Comparison of protomer orientations in the X-ray crystal structure of NS1B CTD and in one of the published X-ray crystal structures (PDB ID: 3EE)(Xia and Robertus, 2010) of the NS1A CTD. (c) The same dimer structure of the NS1A CTD (PDB ID: 3EE) (Xia and Robertus, 2010), which has been confirmed in solution by NMR studies (Aramini et al., 2014; Aramini et al., 2011), with electrostatic surface calculated with APBS (Baker et al., 2001) plugin of Pymol (DeLano, 2002) at ± 5 kT/e (left – same orientation as illustrated by the ribbon diagram inset, right – rotated by 180 deg). The structure of NS1A C-terminal domain does not include a basic surface needed to support RNA binding.