The World Health Organisation (WHO) and the International Consensus Classification (ICC) classify myeloproliferative neoplasms (MPN) into chronic myeloid leukemia (CML), with the characteristic presence of a BCR::ABL1 fusion gene (BCR::ABL1pos), and BCR::ABL1 negative (BCR::ABL1neg) disorders (i.e. essential thrombocythemia [ET], polycythemia vera [PV] and (primary/secondary/pre-fibrotic) myelofibrosis [MF]), which are variably positive for mutations in JAK2 (JAK2 V617F, JAK2 V617Fpos), CALR or MPL in > 90% of patients, commonly leading to activation of the JAK/STAT pathway. The gene discussed is JAK2; the disease is myeloproliferative neoplasm.