The invention relates to ligands that comprise a moiety (e.g., a dAb) that has a binding site with binding specificity for an endogenous target compound but do not substantially inhibit the activity of said endogenous target compound. Preferably, the ligand does not bind to the active site of an endogenous target compound. The invention relates to the use of such a ligand for the manufacture of a medicament for increasing the half-life, bioavailability, activity or amount of an endogenous target compound to which the ligand binds.
