Myeloproliferative neoplasms (MPN) are clonal stem cell disorders characterized by aberrant proliferation of the erythroid, megakaryocytic and myeloid lineages. They are associated with decreased survival, thromboembolic complications, hemorrhage and an inherent tendency towards leukemic transformation. About 4-6 of 100'000 residents per year are diagnosed with MPN Currently, MPN are subdivided into three disease entities: polycythemia vera (PV), essential thrombocythemia (ET) and primary myelofibrosis (PMF). Three genes are frequently mutated in MPN and are implicated to be the phenotypic driver mutations: more than 95% of PV patients carry a somatic JAK2-V617F mutation, while about half of the remaining PV patients (2-3%) display mutations in JAK2 exon 12. Thus, almost all patients with PV have somatic mutations in the JAK2 gene. The mutational profiles of ET and PMF are more diverse: JAK2-V617F is found in 50-60% of the patients, whereas the recently described mutations in calreticulin (CALR) occur in 20-25% of the patients. Between 5-8% of the remaining ET and PMF patients carry mutations in the thrombopoietin receptor (MPL) and in 10-15% of the patient the mutation remains to be determined. Quantitative tests are now available to follow the mutant allele burden of each of the JAK2, CALR and MPL mutant alleles.