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The diagnosis of the myeloproliferative neoplasms (MPN) essential thrombocythaemia (ET) and primary myelofibrosis (PMF) in patients lacking a molecular marker is challenging. Recently, mutations in exon 9 of the calreticulin gene (CALR) have been described in around one-third of ET and MF patients (Klampfl et al, 2013a; Nangalia et al, 2013).

Notably, these mutations are almost always seen in JAK2 V617F-negative and MPL-non-mutated patients and account for the majority of these cases. The prevalence of these mutations has led to the BSH including them in the criteria for the diagnosis of ET and also PMF.

Declaration of Interests

The BSH paid the expenses incurred during the writing of this guidance. None of the authors had conflicts of interest to declare. All authors have made a declaration of interests to the BSH and Task Force Chairs which may be viewed on request.