Evi-1 is a transcription factor that is implicated in leukemic transformation of hematopoietic cells. Two distinct alternative forms, Evi-1a and Evi-1c, are generated from the EVI-1 gene. Whereas Evi-1a is widely recognized as an oncoprotein, a role for Evi-1c, which has an additional PR domain in the amino-terminus of Evi-1a, in leukemogenesis, has not been elucidated thus far. Aberrant oligomerization of transcription factors has recently emerged as a prevalent mechanism for activating their oncogenic potential in hematopoietic malignancies. Here, to study the mechanisms that underlie Evi-1-mediated oncogenesis, we investigated formation of oligomeric complexes by the Evi-1 proteins. We show that Evi-1a forms homo-oligomers, whereas Evi-1c exclusively exists as a monomer in mammalian cells. Remarkably, Evi-1c has lost the ability to interact with CtBP, a transcriptional corepressor that associates with Evi-1a. As a consequence, the ability of Evi-1c to repress transforming growth factor-beta (TGF-beta) signaling is significantly abrogated. These results identify a novel function of a PR domain to regulate oligomerization of transcription factors and suggest that homo-oligomerization may play a critical role in corepressor recruitment by the Evi-1 proteins. In addition, we found that the chimeric oncoprotein acute myelocytic leukemia (AML)1-Evi-1, generated in t(3;21) leukemia, also forms homo-oligomers and hetero-oligomers with Evi-1a, while it did not interact with Evi-1c. Consistent with the results, repression of TGF-beta by AML1-Evi-1 was significantly enhanced by Evi-1a, whereas it was hardly affected by the presence of Evi-1c. These results suggest that oligomerization may contribute to the oncogenic potential of Evi-1-containing proteins.