T EL (also known as ETV6) is a member of the E26 transformation-specific (ETS) family of transcription factors.(1) The highly conserved ETS domain is located at the Cterminal region, while a distinct domain with weak homology to the well-described helix-loop-helix (HLH) domain (also referred to as the pointed domain) is located at the N-terminal region. The former serves for DNA binding to the ETS-binding consensus site (EBS) (GGAA/ T) and the latter for homodimerization and heterodimerization with other ETS family members.(2,3) Through interacting with relevant corepressors mSin3A, N-CoR and SMRT, and histone deacetylase-3, (4) TEL mediates transcriptional repression on its target genes such as FLI-1,inhibitor of differentiation / DNA binding-1 (Id-1), (5) stromelysin-1 (6) and Bcl-X L .(7) Transcriptional activities of TEL are regulated through phosphorylation with mitogen-activated protein kinases (8,9) and small ubiquitin-like modifier conjugation. (10,11) The TEL gene that is mapped to 12p13 is most frequently rearranged and fused to various partner genes by chromosomal translocations in human leukemias and myelodysplastic syndromes. The partners include receptor type or non-receptor type tyrosine kinases and transcription factors. Providing tyrosine kinases, such as platelet-derived growth factor receptor β (PDGFRβ) in t(5;12) (q33;p13), (12) ABL1 in t(9;12) (q34;p13),ARG (ABL2) in t(1;12) (q25;p13), (14) JAK2 in t(9;12) (p24;p13) (15) and Syk in t(9;12) (q22;p13), (16) with the HLH domain, TEL homodimerizes them and thereby stimulates their kinase activities. In contrast, TEL gives corepressor-binding domains to a transcription factor AML1 in t(12;21) (p13;q22) and interferes with its transcriptional abilities.(17) Therefore, dysregulation of the partner proteins by TEL functional domains seems to cause leukemia in patients with 12p13 translocations. Moreover, inactivation of the TEL gene is speculated to be the second hit in t(12;21) (p13;q22) type leukemia, because the wild-type-TEL allele is deleted in the vast majority of the patients. (18,19) Thus, TEL appears to be a tumor suppressor. Consistent with its roles as a putative tumor suppressor, expression of TEL in Ras-transformed NIH3T3 cells inhibits cell growth in liquid and soft agar cultures, (6) and in serum-starved NIH3T3 cells induces apoptosis. (6)