The WHO classification requires a platelet count ≥ 450 × 109 /L, a typical bone marrow morphology, the exclusion of other WHO-defined diseases such as CML, PV and PMF or other myeloid neoplasms, and one of the driver mutations in JAK2, CALR, or MPL. In their absence, the WHO classification calls for the presence of another clonal marker or the exclusion of reactive thrombocytosis (Table 2) [63]. Here, JAK2 is linked to myeloid neoplasm.