Comparison of OR gene similarity among humans, dogs, mice, and pigs by clustering analysis of OR genes on the basis of their amino acid sequence similarity. The names of ORs with known chemical specificity from humans and mice were indicated above the graph. The Y-axis of the upper graph shows the number of OR genes in each cluster ranging from two to 26 genes. The X-axis of both the upper and lower graphs indicates the cluster number, with 706 clusters. The Y-axis of the lower graph indicates the percentage of OR genes of each species within the cluster. The OR genes of different species are indicated by different colors. See the additional file 7 for the uncompressed original image of Figure 3.