Participation of the hairpin element in the binding between the Pol b 3 0 UTR and Hax-1 was demonstrated by UV-cross-linking, in which the transcript with an intact hairpin structure bound to the protein, whereas a transcript with a disrupted hairpin did not. Cross-linking also revealed that Hax-1 binds to mRNA only in the form of a dimer. The presence of a band of a molecular weight of 100 kDa indicates that UV exposure cross-linked a complex consisting of RNA bound to a protein dimer. RNA-monomer complexes were not detected. Data from chemical cross-linking lead us to conclude that the RNA molecule is not necessary for dimerization. However, given that only a small percentage of protein dimerizes in vitro in the absence of mRNA, the possibility that RNA binding influences the dimerization rate is tempting, and remains to be assessed.