The present study may contribute to the development of novel therapeutic strategies against intractable AML with FLT3-ITD, because inhibition of STAT5 was found to synergistically enhance cytotoxic effects of the PI3K/Akt pathway inhibitors by downregulating Mcl-1 expression in AML cells expressing FLT3-ITD. Here, MCL1 is linked to acute myeloid leukemia.