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Axin is a major scaffold protein, interacting with diverse molecules involved in a number of signaling pathways. Axin can undergo dimer/oligomerization via its DIX domain. Here we show that whereas deletion of the DIX domain at the C terminus rendered Axin incapable of forming dimer, a larger deletion of the C-terminal region restored the ability of Axin to form dimers. Detailed analyses revealed that Axin actually contains two separate domains (D and I) in addition to the DIX domain for homodimerization. The D, I, and DIX domains alone can form homodimers. Interestingly, D and I domains strongly interact with each other, suggesting that Axin can form an intramolecular structure through D and I interaction in the absence of DIX. We also found that DIX-DIX homodimeric interaction is weak but that point mutations in the DIX domain abolished Axin homodimerization. We propose a model to suggest that Axin forms …