Analysis of several cases of t(1:14)(p32;q11) translocation present in 3% of T-cell acute leukemias (T-ALL) has revealed the tal-1 gene. This gene encodes a helix-loop-helix protein. It has been found to be expressed in normal bone marrow and in leukemic T-cell and erythroleukemia cell lines, but not in normal T cells. Recently, a site-specific deletion, tald, renamed tald1 in this paper, has been detected in a high proportion of pediatric T-ALL, which arose by a site-specific DNA recombination between tal-1 and a new locus termed SIL. In this study we searched for structural rearrangements within tal-1 in a panel of 134 non-selected leukemic patients (including 66 with T-ALL). Only 6% of patients with T-ALL harbored the tald1 deletion. A second specific deletion termed tald2 was observed in another 6% of T-ALL patients; it involves another site within tal-1 plus the same site as tald1 in the SIL locus. Similarly to tald1 deletion, tald2 junctions harbor structural characteristics that are reminiscent of aberrant recombinase activity. Moreover, we report a detailed analysis of the tal-1 gene structure. Transcription analysis and in vitro translation data are consistent with the differential expression of several TAL-1 protein species containing the HLH motif but differing in their amino terminus. Taken together, our data indicate that t(1;14) translocations and both tald deletions disrupt the 5' part of the tal-1 gene, placing its entire coding sequences under the control of the regulatory elements of the TCR-delta gene or the SIL gene, both of which are normally expressed in T-cell lineage.