The HOX-11 (TCL-3) gene, which is abnormally expressed in the leukemic cells of some patients with T-cell acute lymphoblastic leukemia, is a new member of the homeobox gene family. It is structurally altered by the t(10;14) chromosomal translocation, resulting in head-to-tail juxtaposition of HOX-11 with the T-cell receptor delta-chain gene. In order to understand the normal functions of HOX-11 and its role in T-cell leukemia, we have determined the exon-intron structure of the HOX-11 gene. By using oligonucleotide primers flanking an intron of the HOX-11 gene, we have developed a quantitative reverse transcription-polymerase chain reaction (RT-PCR) assay for the expression of HOX-11. We detected HOX-11 expression in multiple cell lineages including normal T cells and two T-cell lines in which the HOX-11 gene appeared to be unaltered in structure. Our results suggest that deregulation of the endogenous expression of HOX-11 in normal T cells represents an essential step towards the formation of this type of T-cell leukemia.