To confirm that Hax-1 forms a dimer, purified recombinant protein was used for chemical cross-linking with glutaraldehyde. As indicated in Figure 7 , monomers with a molecular mass of around 50 kDa were detected, as were dimers with a molecular mass around 100 kDa. Since dimer formation was detected in an in vitro experiment performed with purified protein, one can conclude that the RNA molecule is not necessary for dimerization. As deduced from the UV-cross-linking experiments, RNA is bound exclusively by dimeric Hax-1, hence the dimerization rate may represent an important factor for RNA-protein complex formation. Glutaraldehyde cross-linking with increasing protein amounts ( Figure 7A ) allowed for the estimation of the monomerdimer equilibrium dissociation constant (K ddim ) at 13.5 mM AE 5.5.