JAK2 and essential thrombocythemia: Ruxolitinib is an oral Janus kinase (JAK) 1/JAK2 inhibitor first approved by the US Food and Drug Administration (FDA) in 2011 for the treatment of adults with intermediate or high-risk myelofibrosis (MF), including primary MF, post-polycythemia vera (PV) MF, and post-essential thrombocythemia MF (Fig. 1) [1].