TEL is a transcriptional repressor that is a frequent target of chromosomal translocations in a large number of hematalogical malignancies. These rearrangements fuse a potent oligomerization module, the SAM domain of TEL, to a variety of tyrosine kinases or transcriptional regulatory proteins. The self-associating property of TEL±SAM is essential for cell transformation in many, if not all of these diseases. Here we show that the TEL±SAM domain forms a helical, head-to-tail polymeric structure held together by strong intermolecular contacts, providing the ®rst clear demonstration that SAM domains can polymerize. Our results also suggest a mechanism by which SAM domains could mediate the spreading of transcriptional repression complexes along the chromosome.