Several antireceptor monoclonal antibodies (MAbs) have been described to compete with ligands for receptor binding. The possibility that structures derived from the hypervariable or complementarity determining regions (CDR) of such antibodies display similarity with those of the ligand binding site of receptors has been documented, thereby allowing the understanding of the structural basis of receptor-ligand interaction. Thus, the determination of the structure of these CDR regions can allow the identification of sequences responsible for the activity of the antibodies.