PROVIDED HEREIN ARE COMPOSITIONS AND METHODS FOR TREATING MYELOFIBROSIS IN A SUBJECT. THE METHODS COMPRISE ADMINISTERING TO THE SUBJECT AN EFFECTIVE AMOUNT OF COMPOUND WHICH IS WHICH IS N-TERT-BUTYL-3 - [(5 -METHYL-2 – {[4-(2-PYRROLIDIN-1-YLETHOXY)PHENYL]AMINO}PYRIMIDIN-4- YL)AMINO]BENZENESULFONAMIDE OR A PHARMACEUTICAL SALT THEREOF OR A HYDRATE THEREOF.