Under normal conditions, JAK2 is a key regulator of hematopoiesis (Quintás-Cardama, Kantarjian, Cortes, & Verstovsek, 2011); however, constitutive activation is associated with PV disease features, including excessive hematopoiesis and splenomegaly (Quintás-Cardama et al., 2011). Here, JAK2 is linked to Splenomegaly.