As the absence of the mutation has been repeatedly confirmed in more than 50% of ET patients, the diagnosis of ET presently remains a mixture of: 1) positive non specific arguments in favor of a Ph-negative MPD, including the JAK2 mutation and the bone marrow (BM) biopsy findings and 2) elimination of PV and IMF according to their currently used and phenotypically based definitions [12,13,20]. This evidence concerns the gene JAK2 and essential thrombocythemia.