The invention relates to the identification of isovaleric acid as a natural ligandof the RCC356 G-protein coupled receptor (GPCR). The invention encompassesthe use of the interaction of RCC356 polypeptides and isovaleric acid as the basisof screening assays for agents that modulate the activity of the RCC356 receptor.The invention also encompasses diagnostic and other assays performed basedupon the RCC356/isovaleric acid interaction, as well as kits for performingdiagnostic and screening assays.