The BCR-ABL1-negative myeloproliferative neoplasms (e.g., essential thrombocythemia, polycythemia vera, and primary myelofibrosis) are a group of heterogeneous hematologic malignancies that involve a clonal proliferation of hematopoietic stem cells. Thrombosis, bleeding, and transformation to acute leukemia reduce the overall survival of patients with myelofibrosis, a disease typified by progressive splenomegaly and disease-related symptoms such as fatigue, pruritus, and bony pains. Hematopoietic stem cell transplant offers the only potential for cure in a minority of eligible patients, leaving a serious unmet need for improved therapies. Recent advances in our understanding of the pathogenetic mechanisms underlying these diseases have led to an explosion of clinical trials evaluating novel therapies. The discovery of an activating mutation in the Janus-activated kinase 2 (JAK2) gene provided a therapeutic target to downregulate this activated signaling pathway, which influences the phenotype of these diseases. Ruxolitinib (Jakafi; Incyte) is a small-molecule inhibitor of JAK1/2 that has proved to be effective at reducing splenomegaly and ameliorating symptoms in myeloproliferative neoplasms. Based on the results of 2 pivotal randomized phase III clinical trials, ruxolitinib has become the first therapeutic to be approved by the U.S. Food and Drug Administration for treatment of patients with myelofibrosis. Ruxolitinib offers a well-tolerated oral therapeutic option for patients with myelofibrosis with symptomatic splenomegaly and debilitating disease-related symptoms, but it does not seem to be effective at eliminating the underlying hematological malignancy.