JAK2 and essential thrombocythemia: A single acquired somatic mutation in the pseudokinase domain of JAK2, in the form of a substitution of Val for Phe at position 617, is at the base of >95% Polycythemia Vera (PV) patients and 50–60% of patients with Essential Thrombocythemia (ET) and Primitive Myelofibrosis (PMF) [15], [16], [17], [18].