Abstract: A novel recombinant human neurokinin-1 receptor short form (hereinafter identified as human NK1R sF) is disclosed which has been prepared by polymerase chain reaction techniques. Also disclosed is the complete sequence of human NK1R sF complementary DNA; expression systems, including a CHO (Chinese hamster ovarian cell line) stable expression system; and an assay using the CHO expression system.NK1R sF, can be used in an assay to identify and evaluate entities that bind substance P receptor or NK1R sF. The assay can also be used in conjunction with diagnosis and therapy to determine the body fluid concentration of substance P antagonists in arthritis patients.