The mutational landscape of ET is similar to PMF with regard to driver mutations: about 50–60% of cases harbor the JAK2 mutation V617F, but no JAK2 exon 12 mutations, up to 30% show CALR and 3–8% MPL mutations; up to 15% are “triple-negative” for the classic MPN driver mutations and are difficult to separate from reactive thrombocytosis (Table 1) [29,34,38,43,44]. Here, CALR is linked to essential thrombocythemia.