Constitutive JAK3 activation has been found in different lymphoid malignancies, including mantle-cell lymphoma [35], Burkitt lymphoma [36], anaplastic large-cell lymphoma [37], HTLV-1-induced adult T-cell lymphoma/leukemia [38], and CTCL [24,39], but the mechanisms of constitutive JAK3 activation are still largely unknown. This evidence concerns the gene JAK3 and leukemia.