The present invention relates to receptor/reporter fusion protein based assays for detecting the effect test compounds have on a particular membrane receptor, as well as to receptor/reporter fusion proteins for use in such assays and compounds identified by the assays as having interesting/useful effects. Suitable membrane receptors are growth factor receptors, cytokine receptors, ion channels and integrins including any subtypes, mutants, homologs and chimeric forms of such receptors. The assay may be to study G-protein coupled receptors (GPCRs).