ZT55 is an orally active and highly-selective JAK2 inhibitor with an IC 50 value of 0.031 μM. ZT55 inhibits the proliferation of JAK2 V617F -expressing HEL cell lines and induces apoptosis and cycle arrest. ZT-55 also effectively inhibits the growth of HEL xenograft tumours in a mice model. ZT-55 can be used in studies of myeloproliferative neoplasms, polycythemia vera and primary thrombocythemia.