A significant gap was present that comprised many cases of MPN that do not harbor any of these mutations, but was recently filled by the discovery of Calreticulin (CALR) mutation in MPNs (6,7). CALR gene mutations are predominantly found in patients with essential thrombocythemia or primary myelofibrosis and are considered to be mutually exclusive with JAK2 and MPL. The gene discussed is CALR; the disease is essential thrombocythemia.