Viral infection requires the insertion of viral genes into host cells. Such a process begins with the binding of coat proteins to host receptors, and in some cases, coreceptors [24] . Ten RNA viruses (seven zoonotic viruses and three non-zoonotic viruses) were investigated. Viruses that use co-receptors were excluded from the study. Receptor sequence data for each virus were collected from the National Center for Biotechnology Information http://www.ncbi.nlm.nih.gov/, and the research literature was examined to determine the specific species tropism of each virus [[25] , http://www.ictvonline.org/]. The viruses, host species, receptors, receptor sequences, and infection information for each host are shown in Table 1 . We selected viruses that are each a representative of a different family, with different primary reservoirs. Viruses with unknown or poorly defined host receptors (particularly human receptors) were excluded from the study. Orthologues of the where N tot is the total number of amino acids in one sequence string; n tot is the total number of matched amino acids in the sequence; N phi and N pho are the numbers of hydrophilic and hydrophobic amino acids in the sequence, respectively; N others is the number of deleted amino acids (gaps/insertions in sequence) plus the number of amino acids with undetermined properties; n phi and n pho are the numbers of hydrophilic and hydrophobic amino acids matched, respectively; and g S i,1 is the similarity score for hydrophilic residues of the i th row of infection group g. Here, there are only three groups: g = 1, 2, or 3, which are the infection, non-infection, and near-infection groups, respectively. The interspecies infection information was identified and classified among three infection states: group 1 (g = 1) represents infection; group 2 (g = 2) represents non-infection; and group 3 (g = 3) represents nearinfection. By definition, if a group 1 species pair includes humans, then the infection is zoonotic. Decisions for grouping were made on the basis of experimental and epidemiological studies reported in the literature [4, [30] [31] [32] [33] .