H protein at Receptor binding site. (a) Distribution of amino acid residues on morbillivirus haemagglutinin protein, showing significant attraction to the signaling lymphocyte activation molecule (SLAM), nectin-4 and for MeV vaccine strains CD46 (modified from Fenggi et al. [88]. (b) Alignment of selected canine distemper virus (CDV) H and phocine distemper (PDV) H amino acid sequences at residues 501–550 in the receptor binding site. Boxes indicate: Red, residues critical for SLAM binding; blue, residue critical for Nectin-4 binding; green, site suggested by McCarthy et al. [70] to be associated with spread to non-canine host. Highlighted residues: Yellow, shared by the Serengeti strains; red, consensus sequence; magenta, differences from the consensus; turquoise specific to monkey virus. Substitutions in PDV have not been highlighted.