Fig. 2.

Phylogenetic tree of sequence relationships among ORs. This tree compares the 339 members of the human OR family, 23 rodent ORs of known function, and 28 fish ORs. Green branches represent fish ORs, and red branches represent human and rodent ORs with known odorant specificities. Odorants detected are indicated near the tip of each red branch. The majority of human homologs of rodent ORs for aliphatic odorants are located in one distinct branch of the tree. This branch (shaded in gray) also contains all of the fish ORs, suggesting a distant evolutionary relationship between receptors for aliphatic odorants and fish ORs.