(a) Neighbour-joining phylogenetic tree containing all functional olfactory receptors (OR) genes in the amphioxus, lamprey, zebrafish and human. Non-OR G-protein coupled receptor (GPCR) genes were used as the outgroup. The Type 2 clade contains several non-OR genes that are not included in group η. Bootstrap values obtained from 500 resamplings are shown for major clades. A scale bar represents the estimated number of amino acid substitutions per site. Adapted from Niimura (2009). (b) Numbers of functional OR genes (including truncated genes) belonging to groups α - η in teleost fish and tetrapods.