Several fusion proteins involving the catalytic active JH1 domain of JAK2 have been reported to be associated with leukemia, as follows: 1) TEL-JAK2 was found in both pre-B-lineage and pre-T-lineage ALL and atypical chronic myelogenous leukemia (CML) [6], [7] 2) PCM1-JAK2 gene fusion resulting from t(8;9)(p22;p24) in eosinophilia-associated atypical CML, ALL, acute myeloid leukemia (AML), and T cell lymphoma [8], and 3) BCR-JAK2 gene fusion as the result of t(9;22)(p24;q11) was found in atypical CML [9], [10] and AML [11]. This evidence concerns the gene PCM1 and acute lymphoblastic leukemia.