Denileukin Diftitox, which is also named “Ontak” or “DAB389 IL-2”, is a fusion protein designed to direct a truncated form of diphtheria toxin to cells that express the high-affinity IL-2 receptor, (consisting of the following subunits: CD25 (IL-2Rα), CD122 (IL-2Rβ), and CD132 (IL-2Rγ)), which is present in many different hematologic malignancies like adult T cell leukemia (ATL), chronic lymphocytic leukemia, Hodgkin’s and non-Hodgkin’s lymphomas, cutaneous T cell lymphoma (CTCL) and other leukemias and lymphomas [10,135,136,137,138,139]. This evidence concerns the gene IL2RG and leukemia.