Ruxolitinib is a selective inhibitor of JAK proteins (Figure 2), in particular of JAK1 and 2, and it is a drug that has been indicated for the pharmacological treatment of myelofibrosis and polycythemia, which belong to the group of myeloproliferative neoplasms, whose pathogenesis is associated with the deregulation of the signals associated with these two enzymes [26]. The gene discussed is JAK1; the disease is myelofibrosis.