“…Interestingly, members of these two families have also been found to be expressed in small numbers in the olfactory epithelium of frogs (Xenopus laevis, Date-Ito et al, 2008), goats (Capra hircus, Wakabayashi et al, 2002;Wakabayashi et al, 2007), mice (Mus musculus, Karunadasa et al, 2006), and humans (Rodriguez et al, 2000). The phylogenetic diversity of these animals indicates that olfactory epithelial expression of V1R and V2R genes may be widespread among tetrapods, suggesting the existence of four classes of olfactory receptor neurons: those expressing ORs, TAARs, V1Rs, and V2Rs.…”