Abstract: Oligopeptides having an amino acid sequence corresponding to a receptor's extracellular domain, and having sequence similarity to regulatory peptides from MHC class I antigens, enhance or replace the physiological response of ligand binding to the corresponding receptor. The oligopeptides are used in diagnosis and therapy of diseases that involve inadequate or inappropriate receptor response as well as in the screening of drug candidates that affect surface expression of receptors. Also useful for drug screening is a modified receptor molecule, where the sequence corresponding to the regulatory peptide is modified or deleted.