The invention disclosed in this patent document relates to transmembrane receptors, particularly to a human G protein-coupled receptor, more particularly to a CXC chemokine receptor subtype 3 (CXCR3), and most particularly to mutated (non-endogenous) version of the human CXCR3 for evidence of constitutive activity and uses thereof. In some embodiments the altered versions of CXCR3 are used, inter alia, for the direct identification of candidate compounds such as receptor agonists, inverse agonists, partial agonist or antagonist for use in, for example and not limitation, graft rejection; inflammatory skin disease; inflammatory bowel disease; allergic inflammation, allergic pulmonary inflammation, inflammatory demyelinating neuropathy, CNS inflammation; rheumatoid arthritis, bronchiolitis obliterans syndrome, periodontal disease and neurodegenerative disease.