Polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF) are myeloproliferative disorders (MPDs) associated with activating mutations of Janus tyrosine kinase 2 (JAK2) gene. The most common mutation, JAK2 V617F, has been reported in ∼97% of patients with PV, ∼50% with ET, and ∼50% with PMF. The resultant JAK2 protein is continuously autophosporylated and therefore always active. It is believed that this mutated tyrosine kinase contributes to the existence and progression of MPDs. CEP701 is an orally available potent low nanomolar inhibitor of wild type and mutated JAK2 tyrosine kinase in enzymatic and cellular assays. Significant inhibition (growth stasis) was observed following CEP-701 subcutaneous administration to V617F-mutated JAK2-dependent HEL.92 xenografts grown in immunocompromised mice. These results indicate that CEP-701 is an attractive candidate for clinical evaluation in patients with MPD carrying a mutated, constitutively activated JAK2. CEP701 is also a potent inhibitor of FLT3 and is being evaluated as FLT3 inhibitor in Phase II/III studies in patients with acute myeloid leukemia, at the starting dose of 80mg PO BID. We designed a Phase II study of CEP701, at the dose of 80 mg PO BID, in patients with PMF and post PV/ET MF, who harbor JAK2 V617F mutation. Eleven patients have been treated so far, seven males, median age 56 years (range, 38–69), median 3 prior therapies (range 0–6); 7 with abnormal cytogenetics; 8 with enlarged spleen (2 had splenectomy); 4 with enlarged liver; 5 transfusion dependent. Five patients have been followed for at least 1 month and have had stable disease. Response will be evaluated using International Working Group for MF Consensus Response Criteria. JAK2 V617F allele burden is being measured monthly. Except for Grade 2 nausea in one patient, no toxicities have been noted so far. Updated results will be presented at the meeting.