Various studies showed that FTY720 had significant antitumorigenic activities against hematologic malignancies including acute lymphoblastic leukemia (ALL), chronic lymphocytic leukemia (CLL) [5], NK-cell leukemia [6], T-cell large granular lymphocyte (LGL) leukemia(T-LGL) [7], Blast crisis of chronic myelogenous leukemia (CML-BC) [8] and AML with c-kit mutation [9]. Here, KIT is linked to aggressive NK-cell leukemia.