BACKGROUND AND OBJECTIVES: Recent data have highlighted an involvement of ABL1 in T-cell acute lymphoblastic leukemia (T-ALL). Specifically, the presence of a fusion gene involving ABL1 and NUP214, both located at 9q34, has been reported. We sought to evaluate whether T-ALL patients with overexpression of ABL showed a peculiar gene expression pattern and were characterized by having specific rearrangements. DESIGN AND METHODS: We previously assessed the expression profile of 128 adults with ALL by oligonucleotide arrays: 33 had T-ALL. In the current study, we evaluated the expression levels of ABL1 in T-ALL cases and found three patients who had ABL1 levels comparable to those detected in BCR/ABL (+)cases and one who had a significantly higher level of ABL1 expression. In order to establish the incidence of ABL1 overexpression in T-ALL, we evaluated 17 additional patients by quantitative (Q)-polymerase chain reaction (PCR) and reverse transcription (RT)-PCR. RESULTS: The three cases with ABL1 expression levels comparable to those found in BCR/ABL (+)cases had a specific signature characterized by a high expression of genes involved in regulation of transcription. The fourth case, with the highest levels of ABL, harbored the NUP214-ABL1 rearrangement, which was confirmed by fluorescence in situ hybridization (FISH). Three of the four patients were refractory to induction chemotherapy. Of the 17 additional patients evaluated by Q-PCR and RT-PCR, none showed ABL1 overexpression. INTERPRETATION AND CONCLUSIONS: Overall, overexpression of ABL1 was found in 8% of T-ALL cases. These results underline the value of microarray analyses for the identification of specific signatures associated with ABL1 overexpression, as well as rearrangements, e.g. NUP214-ABL1, in adult T-ALL.