The invention provides methods of inhibiting epigenetic gene silencing in a cell expressing NPM/ALK or decreasing NPM/ALK content in a cell, by contacting a cell with an agent capable of increasing the concentration of Stat5a protein or its functional analog. Further, the invention provides a method of treating malignancies expressing oncogenic kinase by administering to a patient affected with a malignancy an agent capable of increasing the concentration of Stat5a protein or its epigenetically silenced functional tumor suppressor analog in a malignant cell. Finally, it provides a method to diagnose malignancy and monitor patient's response to therapy by analysis of the degree of DNA methylation of the gene encoding for Stat5a or its analog, their mRNA, or protein.