The nucleoporin 98 gene (NUP98) has been reported to be fused to 13 partner genes in hematological malignancies with 11p15 translocations. Twelve of them have been identified in patients with myeloid neoplasias and only 1, RAP1GDS1 (4q21), is fused with NUP98 in five patients with T-cell acute lymphoblastic leukemia (T-ALL). Three of these patients coexpressed T and myeloid markers, suggesting the specific association of t(4;11)(q21;p15) with a subset of T-ALL originating from an early progenitor, which has the potential to express mature T-cell antigens as well as myeloid markers. We describe here a new NUP98 partner involved in a t(10;11)(q25;p15) in a patient with acute biphenotypic leukemia, showing coexpression of mature T and myeloid markers. The gene involved, located in 10q25, was identified as ADD3 using 3'-RACE. ADD3 codes for the ubiquitous expressed subunit gamma of the adducin protein, and it seems to play an important role in the skeletal organization of the cell membrane. Both NUP98-ADD3 and ADD3-NUP98 fusion transcripts are expressed in the patient. This is the second partner of NUP98 described in T-ALL. Adducin shares with the product of RAP1GDS1, and with all of the nonhomeobox NUP98 partners, the presence of a region with significant probability of adopting a coiled-coil conformation. This region is always retained in the fusion transcript with the NH(2) terminus FG repeats of NUP98, suggesting an important role in the mechanism of leukemogenesis.