JAK2 and essential thrombocythemia: Mutations within JAK2 exon 14 associate primarily with MPNs, where JAK2 p. V617F occurs in >95% of patients with polycythemia vera (PV), and ∼60% of patients with essential thrombocythemia (ET) or primary myelofibrosis (PMF) (Baxter et al., 2005; James et al., 2005; Kralovics et al., 2005; Levine et al., 2005; Vainchenker and Constantinescu, 2013; Silvennoinen and Hubbard, 2015a).