Approximately 90% of patients with MPNs harbor mutations involving the JAK2, CALR, or MPL genes (phenotypic drivers in MPN), resulting in hyperactivation of the Janus-Associated Kinase/Signal Transducers and Activators of Transcription (JAK/STAT) signaling pathway [2–4]. The gene discussed is JAK2; the disease is myeloproliferative neoplasm.