G protein-coupled receptors (GPCRs) are the largest family of cell-surface
receptors, with more than 800 members identified thus far in the human genome.
They regulate the function of most cells in the body, and represent
approximately 3% of the genes in the human genome. These receptors respond to a
wide variety of structurally diverse ligands, ranging from small molecules, such
as biogenic amines, nucleotides and ions, to lipids, peptides, proteins, and
even light. Ligands (agonists and antagonists) acting on GPCRs are important in
the treatment of numerous diseases, including cardiovascular and mental
disorders, retinal degeneration, cancer, and AIDS. It is estimated that these
receptors represent about one third