Compared to other cancers the hematopoietic neoplasms carry strikingly few mutations [46]—in MPNs, almost half of all patients bear only a phenotypic driver mutation and genetic lesions in younger patients below the age of 39 appear to be confined to the phenotypic driver mutations JAK2, CALR, or MPL [47,48]. The gene discussed is JAK2; the disease is cancer.