The invention relates to cells which comprise a membrane receptor which comprises a ligand-binding section, a membrane-localization signal and a mediator section, and which is characterized in that only when there is binding or, alternatively, only when there is a lack of binding of a ligand to the ligand-binding section of the membrane receptor is a structural change brought about with effects on the mediator section to result in binding of an effector protein or polypeptide, which is capable of activating a Ras or Ras-like signal pathway in the cell, to the component of the membrane, where appropriate via other proteins or polypeptides (adaptors). It further relates to assay methods employing these cells, which are used, inter alia, to detect specific interactions between said membrane receptor and a ligand, and kits for use in these assays.