The second amino acid of the DRY motif is preferably Alanine, Aspartate, Glutamate, Histidine, or Asparagine, but may be any amino acid other than Arginine or Lysine, as shown in FIG. 2b. [0092] ???Modified GRK??? means a GRK modified such that it alters desensitization. [0093] ???Naturally occurring GPCR??? means a GPCR that is present in nature. [0094] ???Odorant ligand??? means a ligand compoun