We have analyzed t(1;14)(p32;q11) chromosome translocations from two patients with T cell acute lymphocytic leukemia. The chromosome 1 breakpoints of these patients lie within a kilobasepair of each other, and thus define a genetic locus (designated tal) involved in T cell oncogenesis. Moreover, we have identified sequences within tal that potentially encode an amphipathic helix‐loop‐helix motif, a DNA‐binding domain found in a variety of proteins that control cell growth and differentiation. The homology domain of tal is especially related to that of lyl‐1, a gene on chromosome 19 that has also been implicated in T cell oncogenesis. Hence, tal and lyl‐1 encode a distinct family of helix‐loop‐helix proteins involved in the malignant development of lymphocytes.