Among the major criteria of BCR/ABL1 negative MPN, the presence of variants in driver genes stands out, as well as the Janus kinase 2 gene (JAK2—HGNC: 6192), the thrombopoietin receptor gene (MPL—HGNC: 7217) and the calreticulin gene (CALR—HGNC: 1455). The gene discussed is CALR; the disease is myeloproliferative neoplasm.