JAK2 and essential thrombocythemia: The most frequent mechanism of JAK/STAT activation in MPNs is the point mutation JAK2 p.V617F, which is found in 97% of PV patients, 50–60% of ET patients, and 55–60% of MF patients [3], although the variant allele frequency (VAF) is generally lower for ET patients than PV or MF patients [4,5,6].