Each subset (PV, ET, PMF) has its unique clinical features with a unifying theme of somatic acquisition of a mutation in either Janus Activated Kinase 2 (JAK2V617F) (1–5), Calreticulin (CALR) (6, 7) or Thrombopoietin Receptor (TPOR, MPL) in a Hematopoietic Stem Cell (HSC). Here, CALR is linked to essential thrombocythemia.