Disclosed is a method for indirectly coupling a small molecule ligand to a molecule to be labelled with the ligand, the method comprising the step of: contacting a scaffold molecule, to which is attached at least one small molecule ligand, with the molecule to be labelled, the scaffold molecule having at least one group which is reactive towards a receiver moiety present or formed in situ on the molecule to be labelled, so as to forma bond between the scaffold molecule and the molecule to be labelled, thereby indirectly coupling the small molecule ligand to the molecule to be labelled.