Previously the function of hormone binding proteins has been viewed entirely as one of either sequestering ligand activity or of delivering ligand to target tissues. However, some binding proteins have the ability when complexed with ligand to interact directly with target tissues and can undergo considerable post-translational and post-secretional modifications that serve to modify their action. We propose that for the corticotrophin-releasing factor-binding protein (CRF-BP), this adds a further level at which hormonal action may be regulated. This contrasts with previous concepts of a passive role and shows them as important regulators of hormonal action in their own right.