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Keywords: bleeding disorders, mutation detection, F5F8D. doi:10.1111/j.1365-2141.2007.06809.x Prevalance of JAK2 V617F and exon 12 mutations in polycythaemia vera Identification of the JAK2 V617F mutation has resulted in proposals for a new molecular classification of the myelopro- liferative disorders (MPDs), together with new diagnostic criteria (Campbell & Green, 2006; Tefferi et al, 2007). These propose that, amongst other criteria, presence of a V617F or functionally similar JAK2 mutation, such as those recently described in JAK2 exon 12 (Scott et al, 2007), is required for a diagnosis of polycythaemia vera (PV). However, it remains unclear whether these two mutation types account for all instances of PV. To determine if cases of V617F-negative, exon 12 mutation-negative PV do occur, the JAK2 genotype of patients attending Addenbrooke’s Hospital (Cambridge, UK) was assessed in peripheral blood (PB) and/or granulocyte DNA using allele-specific polymerase chain reaction (PCR) assays (Baxter et al, 2005; Scott et al, 2007). The 114 patients included in this cohort all fulfilled Polycythaemia Vera Study Group (PVSG) diagnostic criteria (Pearson, 2001), and 110 (96%) were positive for the V617F mutation. In one case, the mutant allele was not detectable in total PB DNA but was observed when granulocyte DNA was tested (patient A; Fig 1A), indicating a low level of mutation-bearing cells. A JAK2 exon 12 mutation was detected in three of the four remaining patients, suggesting the incidence of these muta- tions amongst PV patients is approximately 3%. However, this is likely to be an underestimate of their true prevalence, as exon 12 mutations have also been detected in patients who fail to fulfill PVSG criteria, and are consequently labelled as having idiopathic erythrocytosis (Percy et al, 2007; Scott et al, 2007). One case was consistently negative for JAK2 mutations by allele-specific PCR, with tests performed on granulocyte DNA samples obtained 4, 6 and 7 years after diagnosis (patient B; Fig 1A). This 82-year-old female presented with a robust erythrocytosis (haemoglobin, 179 g/l; red cell mass, 171%) and mild thrombocytosis (505 · 10 9 /l), low serum erythropoietin levels (2Æ5 IU/l), and erythropoietin-independent erythroid colonies (EECs), yet had a normal white cell count (10Æ3 · 10 9 /l) Correspondence ª 2007 The Authors Journal Compilation ª 2007 Blackwell Publishing Ltd, British Journal of Haematology, 139, 504–513 511