The tal-1 proto-oncogene encodes a helix-loop-helix DNA-binding protein that has been implicated in the formation of T cell acute lymphoblastic leukemia (T-ALL). Patients with T-ALL harbor structural rearrangements of tal-1 that result from either local DNA deletion or t(1;14)(p34;q11) chromosome translocation. By analyzing t(1;14)(p34;q11) chromosomes from a series of patients, we have now identified a discrete region of tal-1 wherein most of the translocation breakpoints occur. Moreover, mapping of tal-1 genomic DNA revealed that coding exons are situated on both sides of the t(1;14)(p34;q11) major breakpoint region. Hence, the translocated allele of tal-1 is truncated in a manner that reduces its amino acid coding potential.

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