The most important issue with regard to the possibility that leukemia is a feature of the natural history of PV and ET is that not only is mutated JAK2 one of the CHIP or ARCH mutations, but it is also the cognate tyrosine kinase of another oncogene, MPL, and facilitates its oncogenic behavior as well as that of CHIP mutations, such as TP53. This evidence concerns the gene MPL and essential thrombocythemia.