Only the C-terminal part of Hax-1 appears to be involved in mRNA binding, since a truncated peptide bearing only the last 150 aa of the protein suffices for binding the hairpin-containing element, as demonstrated in our experiments utilizing the yeast three-hybrid system ( Figure 5 ). Considering that an interaction with RNA occurs only for dimeric Hax-1, these findings suggest that the domain responsible for the dimerization is also located in the C-terminal part of the protein. From these results, we deduce that BH domains (BH1 and BH2) present in the N-terminal part of the protein do not take part in the dimerization. BH domains and a transmembrane domain (present at the C-terminus of Hax-1) represent the features of Bcl-2 family proteins, but there is no significant sequence homology between these proteins and Hax-1only a weak, partial homology to pro-apoptotic Nip3 (35) . Even though BH domains are known to be important for oligomerization of the proteins from the Bcl-2 family, data seem to exclude the possibility that they are responsible for dimerization of Hax-1.