In a broad aspect the present invention generally relates to novel dimer-complexes (herein called “non-fused-dimers” or NFDs) comprising single variable domains, methods of making these complexes and uses thereof. These non-covalently bound dimer-complexes consist of two identical monomers that each comprises of one or more single variable domains (homodimers) or of two different monomers that each comprises on or more single variable domains (heterodimers). The subject NFDs have typically altered e.g. improved binding characteristics over their monomeric counterpart. The NFDs of the invention may further be engineered through linkage by a flexible peptide or cysteines in order to improve the stability. This invention also describes conditions under which such NFDs are formed and conditions under which the formation of such dimers can be avoided.
