The 2016 WHO category of MPNs includes the three major subcategories of JAK2/CALR/MPL mutation-related MPNs (i.e., PV, ET, and PMF), as well as four other clinicopathologic entities: chronic myeloid leukemia (CML), chronic neutrophilic leukemia (CNL), chronic eosinophilic leukemia, not otherwise specified (CEL-NOS) and MPN, unclassifiable (MPN-U). The gene discussed is CALR; the disease is essential thrombocythemia.