BACKGROUND AND OBJECTIVES: The NK-like homeobox gene (NKX2-5/CSX) plays a crucial role in cardiac development but is not normally expressed in hematopoietic cells. Here, we describe for the first time a fusion between NKX2-5 and the T-cell receptor delta locus (TRD) resulting in NKX2-5 activation in a case of T-cell acute lymphoblastic leukemia (T-ALL). DESIGN AND METHODS: Genomic DNA from a T-ALL patient with an atypical rearrangement, detected by Southern blotting, was analyzed by ligation-mediated polymerase chain reaction (PCR) with TRD-specific primers. Expression of NKX2-5 was analyzed by real-time quantitative PCR in the T-ALL case with the NKX2-5-TRD rearrangement, 18 other cases of T-ALL, three T-ALL derived cell lines, two non-hematopoietic cell lines, peripheral blood mononuclear cells from six healthy individuals and sorted thymocyte subsets. RESULTS: Sequence analysis of ligation-mediated PCR products revealed a novel rearrangement between the third diversity segment of the TRD locus (TRDD3) and a region on chromosome 5q35.1 located 32 kb downstream of the NKX2-5/CSX gene. As a result of this recombination NKX2-5 was placed under influence of the TRD enhancer, resulting in strong ectopic NKX2-5 expression. High NKX2-5 expression was also found in the T-cell lines PEER and CCRF-CEM, which harbor an NKX2-5-BCL11B rearrangement, and in the embryonic kidney cell line 293. NKX2-5 was not expressed in any of the major thymocyte subsets, in normal peripheral blood mononuclear cells, or in the majority (17/18) of the other cases of T-ALL. INTERPRETATION AND CONCLUSIONS: Our finding of overexpression of yet another homeobox gene in T-ALL further supports the hypothesis that homeobox genes play an important role in malignant transformation of particular types of T-ALL.