We analyzed a t(1;14)(p32;q11) chromosomal translocation in a human lymphohemopoietic stem cell line derived from a patient with acute T-lymphoblastic leukemia. The chromosomal joining on the 1p+ chromosome occurred at the T-cell receptor delta diversity (D delta 2) segment, and the reciprocal chromosomal joining on the 14q- chromosome occurred at the T-cell delta diversity segment D delta 1. The involvement of delta diversity segments at the translocation junctions suggests that the translocation occurred during an attempt at D delta 1-D delta 2 joining in a stem cell. The segment of chromosome 1 at band p32, adjacent to the chromosomal breakpoint, encodes a transcriptional unit designated TCL5 (T-cell leukemia/lymphoma 5). The differential expression of the TCL5 RNA transcripts in this lymphohemopoietic stem cell line relative to several other T- and B-cell lines suggests that TCL5 gene expression is an integral event in the pathogenesis of the T-cell leukemia. Rearrangement of the TCL5 locus in a human melanoma cell line carrying a del(1p32) further implies that the TCL5 gene may play a role in malignant transformation.