RSV F phylogeny, subtype sequence alignment, secondary structure definition, and sequence entropy. (A) Molecular phylogenetic analysis of RSV F sequences by Maximum Likelihood. The tree was drawn to scale, with branch lengths measured in the number of substitutions per site. Strains B1, B18537, B9320, and A2, are labeled. (B) Sequence alignment of RSV F proteins from B18537, B9320 and A2. Antigenic sites Ø, I, II, III, and IV are marked above the sequences with color-coded lines. Site III (colored pale yellow) is discontinuous and has substantial overlaps with sites II and IV. Residues that differ between the strains are highlighted in purple and those also found in antigenic sites are highlighted in bold. The DSCav1 mutations (S155C, S190F, V207L, S290C) are highlighted in green. Secondary structural elements of RSV F as defined from the B18537 structure are shown below the sequence alignment. Sequence entropy calculated from 190 RSV F sequences is plotted above the sequence with a color gradient according to the color key shown at the lower right. Residues between 2 arrows (residues 110-136) are removed by protease digestion and therefore not present in the mature proteins.