The NUP214-ABL1 fusion kinase has recently been identified in 6% of patients with T-cell acute lymphoblastic leukemia. In contrast to the more common oncogenic ABL1 fusion BCR-ABL1, NUP214-ABL1 localizes to the nuclear pore complexes and has attenuated transforming properties in hematopoietic cells and in mouse bone marrow transplant models. We have performed a thorough biochemical comparative analysis of NUP214-ABL1 and BCR-ABL1 and show that, despite their common tyrosine kinase domain, the two fusion proteins differ in many critical catalytic properties. NUP214-ABL1 has lowerin vitrotyrosine kinase activity, which is in agreement with the absence of phosphorylation on its activation loop. NUP214-ABL1 was more sensitive to imatinib (Glivec) than BCR-ABL1in vitroand in cells, indicating a different activation state and conformation of the two ABL1 fusion kinases. Using a peptide array, we identified differences in the spectrum and efficiency of substrate peptide phosphorylation and a differential involvement of Src kinases in downstream signaling. These results clearly indicate that different fusion partners of the same kinase can determine not only localization, but also critical functional properties of the enzyme such as inhibitor sensitivity and substrate preference, with subsequent differences in downstream signaling effectors and likely consequences in disease pathogenesis.