JAK2 and essential thrombocythemia: They are often associated with a dysregulated protein tyrosine kinase.1, 2, 3 The JAK2V617F mutation results in constitutive activation of Janus kinase 2 (JAK2) and is associated with over 90% of patients with polycythaemia vera (PV), and about half of patients presenting with myelofibrosis or essential thrombocythaemia.4 Other mutations in JAK2 are found in a minor but significant number of PV cases (<4%) including the JAK2K539L mutation.5 Other mutually exclusive mutations are found in calreticulin and myeloproliferative leukaemia virus oncogene.6, 7, 8