Gene targeting studies in mice have shown that the transcription factor Ikaros plays an essential role in lymphoid development and as a tumor suppressor in T cells, whereas the related gene Aiolos functions as a tumor suppressor in B cells. We analyzed the expression levels of the Ikaros gene family, Ikaros and Aiolos, in human bone marrow samples from patients with adult acute lymphoblastic leukemia [ALL (n = 46; B-cell ALL = 41; T-cell ALL = 5)]. Overexpression of the dominant negative isoform of Ikaros gene Ik-6 was observed in 14 of 41 B-cell ALL patients by reverse transcription-PCR, and the results were confirmed by sequencing analysis and immunoblotting. None of the other dominant negative isoforms of the Ikaros gene were detected by reverse transcription-PCR analysis. Southern blotting analysis with PstI digestion revealed that those patients with the dominant negative isoform Ik-6 might have small mutations in the Ikaros locus. We did not detect any overexpression of dominant negative isoforms of Aiolos in adult ALL patients. These results suggest that Ikaros plays a key role in human B-cell malignancies through the dominant negative isoform Ik-6.