Some PRs are found among the ORs and GRs. ORs form
a receptor protein family encoded by about 60 genes
[24–27], about 40 of which are expressed in small numbers
of OSNs in either the antenna or the MP [28]. The
insect ORs share no similarity to any chemosensory
receptors of mammals or Caenorhabditis elegans. However,
Drosophila ORs and GRs were probably derived
from a common ancestor, because they share some
sequence similarity, especially near the carboxy termini
[25,29–31]. On the basis of their overall structure — 7
hydrophobic stretches that are proposed to form transmembrane
helices — it is believed that ORs and GRs
belong to the G-protein coupled receptor (GPCR) super
family [24–27,29–31].