When employed in clinical trials, JAK2 inhibitors produced significant clinical improvement in patients with myelofibrosis [22], [23] or hydroxyurea-resistant PV or ET [24] at such an extent that the JAK1 and JAK2 inhibitor ruxolitinib has been approved recently by FDA for the treatment of patients with intermediate and high risk myelofibrosis based on the results of two large phase III trials [25], [26] demonstrating rapid and sustained reduction of splenomegaly and improvement of constitutional symptoms. Here, JAK1 is linked to myelofibrosis.