Oncogenic JAK2 with the prominent JAK2V617F mutation found in over 50% of myeloproliferative neoplasia (MPN) patients [56] is the paradigm for the understanding of structure/function relations of JAK activity [57,58,59] and for the general alertness of the cancer field for mutated JAK family members as potential oncogenes. The gene discussed is JAK2; the disease is cancer.