JAK2 and essential thrombocythemia: In 2005, a mutation at base 1849 in exon 14 of the JAK2 gene on chromosome 9 was discovered in patients with polycythaema vera, essential thrombocythaemia and primary myelofibrosis.2,3,4 This somatic point mutation causes a substitution of guanine by thymine and the amino acid is changed from valine to phenylalanine in codon 617 of the JAK2 protein.2 This discovery has improved understanding of the pathophysiology of MPNs and has renewed interest and research in MPN biology and genetics.5,6