We analyzed a t(1;14)(p32;qll) chromosomal translocation in a human lymphohemopoietic stem cell line derived from a patient with acute T-lymphoblastic leukemia.The chromosomal joining on the lp+ chromosome occurred at the T-cell receptor 8 diversity (D82) segment, and the reciprocal chromosomal joining on the 14q-chromosome occurred at the T-cell 8 diversity segment D8,. The involvement of 8 diversity segments at the translocation junctions suggests that the translocation occurred during an attempt at DS1-DS2 joining in a stem cell. The segment of chromosome 1 at band p32, adjacent to the chromosomal breakpoint, encodes a transcriptional unit designated TCLS (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(lp32) further implies that the TCL5 gene may play a role in malignant transformation.